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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.
【C1】
选项 A、although
B、as
C、but
D、while
答案C
解析 考查逻辑关系。空格前讲的是“人的嗅觉没有动物灵敏”，空格后讲的是“这主要是因为我们是直立的”。由本段首句“人的鼻子是一个被低估的工具”可知，作者并不认同“人的嗅觉没有动物的灵敏”这一普遍认同的观点。由此可推知，空格前后部分存在一种隐约的转折关系，即作者认为人的鼻子是灵敏的，只不过因为人是直立的，所以不能更好地去感知气味。故正确答案为C．but。
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