完形填空（共15小题；每小题2分，满分30分）
阅读下面短文，掌握其大意，然后从下列各题所给的四个选项（A、B、C和D）中，选出最佳选项，并在答题卡上将该项涂黑。
The human nose is an underestimated tool. Humans are often thought to be  1___ smellers compared with animals, but this is largely because, unlike animals, we stand upright. This means that our noses are  2 to detecting those smells which float through the air,  3  the majority of smells which stick to surfaces. In fact though, we are extremely sensitive to smells, even if we do not generally realize it. Our noses are capable of  4 human smells even when these are  5  to far below one part in one million.
　　  6    , some people find that they can smell one type of flower but not another, while others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate  7    smell receptors in the nose. These receptors are the cells which sense smells and send  8   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  9    to it often enough.
　　The  10  for insensitivity to smell seems to be that brain finds it  11   to keep all smell receptors working all the time but can  12  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  13    of the usual smell of our own house but we  14    new smells when we visit someone else's. The brain finds it best to keep smell receptors _ 15   for unfamiliar and emergency signals such as the smell of smoke, which might indicate the danger of fire.

1. A. sensitive	B. outstanding	C. insensitive	D. awkward
2. A. limited	B. committed	C. devoted	D. conducted
3. A. catching	B. ignoring	C. missing	D. tracking
4. A. distinguishing	B. discovering	C. determining	D. detecting
5. A. reduced	B. reserved	C. rescued	D. refused
6. A. Fortunately	B. Strangely	C Happily	D. Amazingly
7. A. unusual	B. particular	C. unique	D. typical
8. A. signs	B. information	C. messages	D. signals
9. A. subjected	B. left	C. drawn	D. exposed
10A. expectation	B. expression	C. extension	D. explanation
11.A convenient	B. competitive	C. inefficient	D. adequate
12.A introduce	B. gather	C. develop	D. produce
13.A sure	B. sick	C. aware	D tired
14.A tolerate	B. resist	C. neglect	D. notice
15.A available	B. reliable	C. valuable	D. suitable