The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, __1__ this is largely because, __2__ animals, we stand upright. This means that our noses are __3__ to perceiving those smells which float through the air, __4__ the majority of smells which stick to surfaces. In fact, __5__, we are extremely sensitive to smells, __6__ we do not generally realize it. Our noses are capable of __7__ human smells even when these are __8__ to far below one part in one million. Strangely, some people find that they can smell one type of flower but not another, __9__ others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate __10__ smell receptors in the nose. These receptors are the cells which sense smells and send __11__ to the brain. However, it has been found that even people insensitive to a certain smell __12__ can suddenly become sensitive to it when __13__ to it often enough. The explanation for insensitivity to smell seems to be that brain finds it __14__ to keep all smell receptors working all the time but can __15__ new receptors if necessary. This may __16__ explain why we are not usually sensitive to our own smells we simply do not need to be. We are not __17__ of the usual smell of our own house but we __18__ new smells when we visit someone else’s. The brain finds it best to keep smell receptors __19__ for unfamiliar and emergency signals __20__ the smell of smoke, which might indicate the danger of fire.
1.[A] although [B] as [C] but [D] while
2.[A] above [B] unlike [C] excluding [D] besides
3.[A] limited [B] committed [C] dedicated [D] confined
4.[A] catching [B] ignoring [C] missing [D] tracking
5.[A] anyway [B] though [C] instead [D] therefore
6.[A] even if [B] if only [C] only if [D] as if
7.[A] distinguishing [B] discovering [C] determining [D] detecting
8.[A] diluted [B] dissolved [C] determining [D] diffused
9.[A] when [B] since [C] for [D] whereas
10.[A] unusual [B] particular [C] unique [D] typical
11.[A] signs [B] stimuli [C] messages [D] impulses
12.[A] at first [B] at all [C] at large [D] at times
13.[A] subjected [B] left [C] drawn [D] exposed
14.[A] ineffective [B] incompetent [C] inefficient [D] insufficient
15.[A] introduce [B] summon [C] trigger [D] create
16.[A] still [B] also [C] otherwise [D] nevertheless
17.[A] sure [B] sick [C] aware [D] tired
18.[A] tolerate [B] repel [C] neglect [D] notice
19.[A] available [B] reliable [C] identifiable [D] suitable
20.[A] similar to [B] such as [C] along with [D] aside from
参考答案：
1-5. [C]. [B]. [A]. [C]. [B] 6-10. [A]. [D]. [A]. [D]. [B]
11-15. [C]. [A]. [D]. [C]. [D] 16-20. [B]. [C]. [D]. [A]. [B]
