The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 【C1】______ 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, 【C1】______ this is largely because,【C2】______animals, we stand upright. This means that our noses are【C3】______ to perceiving those smells which float through the air, 【C4】______the majority of smells which stick to surfaces. In fact, 【C5】______, we are extremely sensitive to smells, 【C6】______ we do not generally realize it. Our noses are capable of【C7】______human smells even when these are【C8】______ to far below one part in one million.
    Strangely, some people find that they can smell one type of flower but not another,【C9】______others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate【C10】______smell receptors in the nose. These receptors are the cells which sense smells and send【C11】______ to the brain. However, it has been found that even people insensitive to a certain smell【C12】______can suddenly become sensitive to it when【C13】______ to it often enough.
    The explanation for insensitivity to smell seems to be that brain finds it【C14】______to keep all smell receptors working all the time but can【C15】______new receptors if necessary. This may 【C16】______explain why we are not usually sensitive to our own smells we simply do not need to be. We are not【C17】______of the usual smell of our own house but we【C18】______ new smells when we visit someone else’s. The brain finds it best to keep smell receptors【C19】______for unfamiliar and emergency signals【C20】______the smell of smoke, which might indicate the danger of fire.
【C15】

选项 A、introduce
B、summon
C、trigger
D、create

答案D

解析 语义衔接题。but前说“大脑发现让所有的气味感应器持续工作效率低”,暗含不让所有感应器都持续工作之意,but表示转折,可见横线所在部分应表示“让感应器工作”。所填动词应能与new receptors构成动宾搭配,new表明,这些感应器是新的,create(创造)符合此处搭配要求,代人空中是说“大脑在必要的情况下创造新的感应器”,可与the brain finds it…all the time构成转折,故[D]项为答案。introduce强调“从外部引进原来没有的事物”,可与new receptors构成动宾搭配,但“大脑从外部引进感应器”不符合逻辑,故排除。summon(召集,召唤)和trigger(引发,激发)后加已有的事物作宾语,与new语义矛盾,故均排除。
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