单选题
The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals,
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this is largely because,
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animals, we stand upright. This means that our noses are
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to perceiving those smells which float through the air,
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the majority of smells which stick to surfaces. In fact,
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, we are extremely sensitive to smells,
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we do not generally realize it. Our noses are capable of
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human smells even when these are
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to fax below one part in one million.
Strangely, some people find that they can smell one type of flower but not another,
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others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate
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smell receptors in the nose. These receptors axe the cells which sense smells and send
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to the brain. However, it has been found that even people insensitive to a certain smell
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can suddenly become sensitive to it when
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to it often enough.
The explanation for insensitivity to smell seems to be that the brain finds it
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to keep all smell receptors working all the time but can
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new receptors if necessary. This may
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explain why we are not usually sensitive to our own smells--we simply do not need to be. We are not
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of the usual smell of our own house, but we
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new smells when we visit someone else"s. The brain finds it best to keep smell receptors
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for unfamiliar and emergency signals
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the smell of smoke, which might indicate the danger of fire.
【答案解析】[解析] 上文提到人类嗅觉不灵敏是由于人类是直立的，本句进一步解释说明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。选A。
[点睛] be committed to表示give one"s whole loyalty to，有“尽职，忠诚、做出明确许诺的”等含义；be dedicated to表示be very interested in work very hard for，有“热忱的”等含义，故B、C均不合题意。 confine...to...表示keep within limits，restrict．有 “把......局限/限制于”等含义，如：Confine your re- marks to the subject：under discussion．limit...to... 表示keep within a certain size，amount，number， area，or place；restrict它一般用于具体范围的“局 限、限制”。本题用limit较好，选A项。
【正确答案】
C
【答案解析】[解析] 此处空格与上文中的perceiving构成对应关系，空格的意思应为“没有感觉到”。A、D项不 合题意。B项意为“忽略，忽视”，含一种主观上的 因素。故选择C项missing，表达failing to detect(未察觉)的含义。
[点睛] 语义逻辑题，兼考查词义辨析。
【答案解析】[解析] 本题所填的形容词用于修饰空格后的to keep au smell receptors working all the time， 从四个选项看，只有inefficient最符合题意，说明 让所有感受器一直工作“效率太低”。
[点睛] 词义辨析题。ineffective意为“无效的、不起作用的”．incompetent意为“不能胜任的”insufficient意为“不充足的”。
【正确答案】
D
【答案解析】[解析] new receptors本来是没有的(所以对某些花香不敏感)，经与刺激物经常接触而产生，从而可以清晰感受到这些刺激。这是从无到有的过 程，D项create最合适。
[点睛] 词义辨析题。虽然“trigger the receptors激活受体”是英语中常见的搭配，但所谓“激活”应该是对现已存在的(但没有起作用的)事物而言。既然大脑中并不 存在这种气味的受体，“激活”也就无从谈起。因此C项 “trigger激活，触发”不对。A项“介绍”，B项“唤起”，均 不合题意。