The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, (51) this is largely because, unlike animals, we stand upright. This means that our noses are (52) to perceiving those smells which float through the air, missing the majority of smells which stick to surfaces. In fact, (53) , we are extremely sensitive to smells, even if we do not generally realize it. Our noses are capable of (54) human smells 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, (55) others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate (56) smell receptors in the nose. These receptors are the cells which sense smells and send (57) 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 (58) to it often enough.
The explanation for insensitivity to smell seems to be that the brain finds it (59) to keep all smell receptors working all the time but can (60) new receptors if necessary.
51().
A.although
B.as
C.but
D.while
参考答案：C
解析：
本题考查逻辑关系。空格前面的句子陈述了一种普遍存在的看法：“和动物相比，人类的嗅觉不灵敏”；空格后面部分则是在比较人类和动物的差异，指出：这只不过是因为人是直立的。这说明，前后句子的意思对应，所以空格处应该填入表示转折关系的词。A．although表示让步。B．as表示原因，在语文上与this is largely because重复。C．but表示转折。D．while有时相当于although，显然不对；有时表示转折，常用来表示同一事件相对的两个方面，而空格前后的句子不是对比关系，所以也不对。只有C．but符合句意。