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  3. The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 【C1】______this

The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 【C1】______this

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问题     The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 【C1】______this is largely because, 【C2】______animals, we stand upright. This means that our noses are【C3】______to perceiving those smells which float through the air,【C4】______the majority of smells which stick to surfaces. In fact,【C5】______, we are extremely sensitive to smells, 【C6】______we do not generally realize it. Our noses are capable of 【C7】______human smells even when these are【C8】______to far below one part in one million.
    Strangely, some people find that they can smell one type of flower but not another, 【C9】______others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate【C10】______smell receptors in the nose. These receptors are the cells which sense smells and send【C11】______to the brain. However, it has been found that even people insensitive to a certain smell【C12】______can suddenly become sensitive to it when【C13】______to it often enough.
    The explanation for insensitivity to smell seems to be that the brain finds it【C14】______to keep all smell receptors working all the time but can【C15】______new receptors if necessary. This may【C16】______explain why we are not usually sensitive to our own smells—we simply do not need to be. We are not【C17】______of the usual smell of our own house, but we【C18】______new smells when we visit someone else’ s. The brain finds it best to keep smell receptors【C19】______for unfamiliar and emergency signals【C20】______the smell of smoke, which might indicate the danger of fire.
【C15】
选项 A、introduce
B、summon
C、trigger
D、create
答案D
解析 考查动词辨析。空格所在部分讲的是“但是在需要时大脑可______新的嗅觉感受器”。由上文可知，大脑为了避免效率低下不让所有嗅觉感受器一直工作。由but可知，空格所在部分和上文是转折关系，因此，空格处所填单词应该“让嗅觉感受器工作”起来。D．create“创造，创作”，符合文意，即“但是在需要时，大脑可以创建新的嗅觉感受器”。summon“召唤”，主语一般是人，不符合文意。另外，大脑也不可能introduce“推行”或是trigger“引起”新的嗅觉感受器，因此均被排除。
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