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.
【C6】

选项 A、even if
B、if only
C、only if
D、as if

答案A

解析 逻辑衔接题。In fact,______,we are extremely sensitive to smells,______ we do not generally realize it(______ 我们通常意识不到,______实际上我们人类嗅觉非常灵敏)表明,横线前后内容意思矛盾,故横线处所填短语构成转折关系,因此答案为even if(即使,虽然)。if only意为“要是……多好”,后接虚拟语气;only if(只有在……情况下)强调条件;as if(好像,仿佛)后接虚拟语气,这三项均不符合此处的逻辑要求,故均排除。
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