A. limited B. distinguish C. unlike D. particular E. triggerF. inefficient G. c

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问题 A. limited B. distinguish C. unlike D. particular E. trigger
F. inefficient G. create H. emergency I. incompetent J. detecting
K. aware L. messages M. unusual N. surfaces O. confined
The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, but this is largely because,【C1】______ animals, we stand upright. This means that our noses are【C2】______to perceiving those smells which float through the air, missing the majority of smells which stick to【C3】______. In fact, though, we are extremely sensitive to smells, even if we do not generally realize it. Our noses are capable of【C4】______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【C5】______smell receptors in the nose. These receptors are the cells which sense smells and send【C6】______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 exposed 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【C8】______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【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. [br] 【C2】

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答案 A

解析空格前的are表明此处需要填入动词的过去分词形式或形容词。上文提到人类嗅觉不灵敏是由于人类是直立的，由missing the majority of smells可推知，这种现象导致人的鼻子只“限于”感觉到漂浮在空气中的气味。limit to“限于”符合语义。confine…to…有“把……局限／限制于”等含义，但一般用于具体范围的“局限、限制”。

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