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  3. The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 【B1】______this

The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 【B1】______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, 【B1】______this is largely because, 【B2】______animals, we stand upright. This means that our noses are【B3】______to perceiving those smells which float through the air,【B4】______the majority of smells which stick to surfaces. In fact,【B5】______, we are extremely sensitive to smells, 【B6】______we do not generally realize it. Our noses are capable of 【B7】______human smells even when these are【B8】______to far below one part in one million.
    Strangely, some people find that they can smell one type of flower but not another, 【B9】______others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate【B10】______smell receptors in the nose. These receptors are the cells which sense smells and send【B11】______to the brain. However, it has been found that even people insensitive to a certain smell【B12】______can suddenly become sensitive to it when【B13】______to it often enough.
    The explanation for insensitivity to smell seems to be that the brain finds it【B14】______to keep all smell receptors working all the time but can【B15】______new receptors if necessary. This may【B16】______explain why we are not usually sensitive to our own smells—we simply do not need to be. We are not【B17】______of the usual smell of our own house, but we【B18】______new smells when we visit someone else’ s. The brain finds it best to keep smell receptors【B19】______for unfamiliar and emergency signals【B20】______the smell of smoke, which might indicate the danger of fire.
【B16】
选项 A、still
B、also
C、otherwise
D、nevertheless
答案B
解析 考查副词辨析。空格所在句讲“这可以______解释为什么我们总是对自己的气味不敏感——只不过是不需要而已”。由文意可知，其中“this”指的是上一句中提到的大脑工作原理，即“大脑觉得，使所有嗅觉感受器一直工作效率低下”。由此可知，和上一句提到的“嗅觉不敏感的原因”一样，“我们总是对自己的气味不敏感”的原因是用大脑工作的这个原理来解释。因此，空格所在句和上句为并列关系，B．also“也”，符合文意。
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