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.
57().
A.signs
B.stimuli
C.messages
D.impulses
参考答案：C
解析：
A．sign：标志，指显示事实、情况或性质在场或存在的某物，例：There are no signs of life about the house．这房子没有有人住的迹象。B．stimulus：刺激因素，指能引起或被认为引起反应的东西，例：Light is a stimulus to growth in plants．光是植物生长的促进因素。C．message：信息，指以书面、信号或其他方式从一个人、一个地方或一群人传给另一人、另一个地方或另一群人的消息，例：I have sent mot：her a message to tell her I shall be home late．我已经给母亲捎了个信儿，告诉她我回家晚点儿。D．impulse：本能，指驱动力或倾向。例：She bought the dress on impulse．她一时冲动买了这件衣服。只有“message”能够与“send”搭配使用。