I. Use of English (Cloze)

    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 the 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] dispersed              [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