The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals,【C11】______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,【C11】______this is largely, because, unlike animals, we stand upright. This means that our noses are【C12】______to perceiving those smells which float through the air, missing the majority of smells which stick to surfaces. In fact,【C13】______, we are extremely sensitive to smells, even if we do not generally realize it. Our noses are capable of【C14】______human smells even when these are diluted to far below one part in one million.
    Strangely, some people find that they can smell one type of flower but not another,【C15】______others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate particular smell receptors in the nose. These receptors are the cells which sense smells and send【C16】______to the brain. However, it has been found that even people insensitive to a certain smell at first can suddenly become sensitive to it when【C17】______to it often enough.
    The explanation for insensitivity to smell seems to be that the brain finds it inefficient to keep all smell receptors working all the time but can【C18】______new receptors if necessary. This may also explain why we are not usually sensitive to our own smells—we simply do not need to be. We are not【C19】______of the usual smell of our own house, but we notice new smells when we visit someone else’ s. The brain finds it’ s best to keep smell receptors【C20】______for unfamiliar and emergency signals such as the smell of smoke, which might indicate the danger of fire.
【C11】

选项 A、although
B、as
C、but
D、while

答案C

解析 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。
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