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.
【C12】

选项 A、limited
B、committed
C、dedicated
D、confined

答案A

解析 空格所在部分讲“这意味着我们的鼻子——感知在空气中飘过的气味”。由上句可知,和动物不同,人类是直立的,由此可知,人的鼻子只能限于感知在空气中飘过的气味。因此,空格处应填表示“局限”的词。limit和confine都表示“局限”,但confine“限制”,表示在一定活动、领域和地域内受到限制。limit“局限于”,表示时间、空间、程度、数量等方面所能或允许达到的最大程度,符合文意,故选A。
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