Passage 2
The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, __1__this is largely because, unlike animals, we stand upright. This means that our noses are__2__to perceiving those smells which float through the air, __3__the majority of smells which stick to surfaces. In fact__4__, we are extremely sensitive to smells, even if we do not generally realize it. Our noses are capable of__5__human smells even when these are__6__to far below one part in one million.
Strangely, some people find that they can smell one type of flower but not another, __7__others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate__8__smell receptors in the nose. These receptors are the cells which sense smells and send __9__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__10__ to it often enough.
The explanation 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__13__explain why we are not usually sensitive to our own smells we simply do not need to be. We are not aware 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 availabe for unfamiliar and emergency signals__15__the smell of smoke, which might indicate the danger of fire.
1. A. although B. as C. but D. while
2. A. limited B. committed C. dedicated D. confined
3. A. catching B. ignoring C. missing D. tracking
4. A. anyway B. though C. instead D. therefore
5. A. distinguishing B. discovering C. determining D. detecting
6. A. diluted B. dissolved C. determining D. diffused
7. A. when B. since C. for D. whereas
8. A. unusual B. particular C. unique D. typical
9. A. signs B. stimuli C. messages D. impulses
10. A. subjected B. left C. drawn D. exposed
11. A. ineffective B. incompetent C. inefficient D. insufficient
12. A. introduce B. summon C. trigger D. create
13. A. still B. also C. otherwise D. nevertheless
14. A. tolerate B. repel C. neglect D. notice
15. A. similar to B. such as C. along with D. aside from
