According to conventional theory, yawning takes place when people are bored or sleepy and serves the function of increasing alertness by reversing, through deeper breathing, the drop in blood oxygen levels that are caused by the shallow breathing that accompanies lack of sleep or boredom. — 根据传统理论，当人们无聊或者困倦时就会打哈欠，伴随睡眠不足或无聊而来的浅呼吸会降低血液中氧的含量，打哈欠可以通过深呼吸逆转血液中氧含量的下降，从而达到提高警觉性的作用。 ① conventional /kənˈvenʃənl/ a. 传统的，惯例的 ② boredom /ˈbɔːrdəm/ n. 乏味；厌倦，厌烦

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Essentially, a theory is an abstract, symbolic representation of what is conceived to be reality.

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Thanks to modern irrigation, crops now grow abundantly in areas where once nothing but cacti and sagebrush could live.

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Anthropologists have discovered that fear, happiness, sadness, and surprise are universally reflected in facial expressions.

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Of the millions who saw Haley’s Comet in 1986, how many people will live long enough to see it return in the twenty-first century.

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The greater the population there is in a locality, the greater the need there is for water, transportation, and disposal of refuse.

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Perfectly matched pearls, strung into a necklace, bring a far higher price than the same pearls sold individually.

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The development of mechanical timepieces spurred the search for more accurate sundials with which to regulate them.

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Studies of the gravity field of the Earth indicate that its crust and mantle yield when unusual weight is placed on them.

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Quails typically have short rounded wings that enable them to spring into full flight instantly when disturbed in their hiding places.

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Fungi are important in the process of decay, which returns ingredients to the soil, enhances soil fertility, and decomposes animal debris.

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Most substance contract when they freeze so that the density of a substance’s solid is higher than the density of its liquid.

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By the middle of the twentieth century, painters and sculptors in the United States had begun to exert a great worldwide influence over art.

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Although apparently rigid, bones exhibit a degree of elasticity that enables the skeleton to withstand considerable impact.

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Finally, the coughing reflex in reaction to irritants in the airway produces not a cough during sleep but a cessation of breathing.

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But when consumers do not know they are being lobbied, they may accept claims they would otherwise be suspicious of.

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Buyers and sellers should be aware of new developments in technology which can and does affect marketing activities.

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With modern offices becoming more mechanized, designers are attempting to personalize them with warmer, less severe interiors.

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The worker and soldier castes of termites consist of both males and females, and the queen lives permanently with a male consort.

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Children with parents whose guidance is firm, consistent, and rational are inclined to possess high levels of self-confidence.

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Lucretia Mott’s influence was so significant that she has been credited by some authorities as the originator of feminism in the United States.

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No social crusade aroused Elizabeth Williams’ enthusiasm more than the expansion of educational facilities for immigrants to the United States.

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Since wind-pollinated flowers have no need to attract insects or other animals, they have dispensed with bright petals, nectar, and scent.

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In the absence of a tradition of classical stone-carving and building, the desire to develop Roman amenities would have been difficult to fulfill.

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Out in remote areas of the countryside, in rural locations, music was more traditional; the same songs were enjoyed by previous generations.

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In the second half of the millennium, in the south around the city of Uruk, there was an enormous escalation in the area occupied by permanent settlements.

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By fossils, we mean traces of prehistoric animals such as bones, which become mineralized, or impressions of bones or organs that are left in stone.

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The application of electronic controls made possible by the microprocessor and computer storage have multiplied the uses of the modern typewriter.

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Even though the precise qualities of hero in literary words may vary over time, the basic exemplary function of the hero seems to remain constant.

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When Henry Ford first sought financial backing for making cars, the very notion of farmers and clerks owning automobiles was considered ridiculous.

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Many people who wanted to become archaeologists ended up pursuing other careers and contributing to archaeological research only as unpaid amateurs.

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Even before the discovery of germs, beliefs that disease spread by “miasmas” （noxious forms of bad air） prompted attention to sewers and open garbage.

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Since the consumer considers the best fruit to be that which is the most attractive, the grower must provide products that satisfy the discerning eye.

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With no oceans to dissolve it, outgassed carbon dioxide began to accumulate in the atmosphere, intensifying the greenhouse effect even more.

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Buildings contribute to human life when they provide shelter, enrich space, complement their site, suit the climate, and are economically feasible.

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Research into the dynamics of storms is directed toward improving the ability to predict these events and thus to minimize damage and avoid loss of life.

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Their streamlined bodies, the absence of hind legs, and the presence of a fluke and blowhole cannot disguise their affinities with land dwelling mammals. ① streamline /ˈstriːmlaɪn/ vt. 使成流线型 ② hind /haɪnd/ a. 后面的，在后的 ③ presence /ˈprezns/ n. 存在；出现

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Scientists do not know why dinosaurs became extinct, but some theories postulate that changes in geography, climate, and sea levels were responsible.

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The elimination of inflation would ensure that the amount of money used in repaying a loan would have the same value as the amount of money borrowed.

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Futurism, an early twentieth-century movement in art, rejected all traditions and attempted to glorify contemporary life by emphasizing the machine and motion.

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Acids are chemical compounds that, in water solution, have a sharp taste, a corrosive action on metals, and the ability to turn certain blue vegetable dyes red.

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In other instances, such as the Indus River Valley, the cities lacked a royal precinct and the ostentatious palaces that marked their contemporaries elsewhere.

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Offshore drilling platforms extend the search for oil to the ocean’s continental shelves—those gently sloping submarine regions at the edges of the continents.

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Even those artists who were most dependent on photography became reluctant to admit that they made use of it, in case this compromised their professional standing.

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According to anthropologists, the earliest ancestors of humans that stood upright resembled chimpanzees facially, with sloping foreheads and protruding brows.

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Anarchism is a term describing a cluster of doctrines and attitudes whose principal uniting feature is the belief that government is both harmful and unnecessary.

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A rotary engine attached to the steam engine enabled shafts to be turned and machines to be driven, resulting in mills using steam power to spin and weave cotton.

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Self-awareness, even in its earliest stages, might entail an awareness of others, the ability to see their perspective and to look at the world from another’s point of view.

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Various committees acted as an executive branch, implementing policies of the assembly and supervising, for instance, the food and water supplies and public buildings.

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