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  3. The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, but this is lar

The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, but this is lar

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问题     The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, but this is largely because, unlike animals, we stand upright This means that our noses are【C1】______to perceiving those smells which float through the air, missing the majority of smells which stick to surfaces. In fact, though, we are【C2】______sensitive to smells, even if we do not generally realize it. Our noses are capable of【C3】______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, whereas others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate【C4】______smell receptors in the nose. These receptors are the cells which sense smells and send【C5】______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【C6】______to it often enough.
    The explanation for insensitivity to smell seems to be that the brain finds it【C7】______to keep all smell receptors working all the time but can create new receptors if necessary. This may also explain why we are not usually sensitive to our own smells—we【C8】______do not need to be. We are not【C9】______of the usual smell of our own house, but we notice new smells when we visit someone else’s. The brain finds it best to keep smell receptors available for unfamiliar and【C10】______signals such as the smell of smoke, which might indicate the danger of fire.
A)simply B)emergency C)aware D)detecting
E)permanently F)limited G)sure H)inefficient
I)dedicated J)exposed K)impulses L)messages
M)distinguishing N)particular O)extremely
【C2】
选项
答案O
解析 从句子结构来看，这里不缺少主要成分，可以判断此空应填人副词修饰sensitive。上句说人类嗅觉不灵敏，空格前的though表转折，提示事实上并非如此。extremely带人句中表示“我们的嗅觉是十分灵敏的”，符合句意。
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