The human nose is an underrated tool. Humans are often thought to be insensitive smelters compared with animals, 21 this is largely because, 22 animals, we stand upright. This means that our noses are 23 to perceiving those smells which float through the air, 24__ the majority of smells which stick to surfaces. In fact, __25__, we are extremely sensitive to smells, __26 we do not generally realize it. Our noses are capable of 27 human smells even when these are 28 to far below one part in one million.Strangely, some people find that they can smell one type of flower but not another, __ 29__ others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate 30 smell receptors in the nose, These receptors are the cells which sense smells and send 31 to the brain, However, it has been found that even people insensitive to a certain smell 32 can suddenly become sensitive to it when 33 to it often enough.The explanation for insensitivity to smell seems to be that brain finds it 34 to keep all smell receptors working all the time but can 35 new receptors if necessary. This may __36__ explain why we are not usually sensitive to our own smells—we simply do not need to be. We are not 37 of the usual smell of our own house but we 38 new smells when we visit someone else’s. The brain finds it best to keep smell receptors 39 for unfamiliar and emergency signals __40__ the smell of smoke, which might indicate the danger of fire.

A.although B.as C.but D.while
问题2：
A.above B.unlike C.excluding D.besides
问题3：
A.limited B.committed C.dedicated D.confined
问题4：
A.catching B.ignoring C.missing D.tracking
问题5：
A.anyway B.though C.instead D.therefore
问题6：
A.even if B.if only C.only if D.as if
问题7：
A.distinguishing B.discovering C.determining D.detecting
问题8：
A.diluted B.dissolved C.dispersed D.diffused
问题9：
A.when B.since C.for D.whereas
问题10：
A.unusual B.particular C.unique D.typical
问题11：
A.signs B.stimuli C.messages D.impulses
问题12：
A.at first B.at all C.at large D.at times
问题13：
A.subjected B.left C.drawn D.exposed
问题14：
A.ineffective B.incompetent C.inefficient D.insufficient
问题15：
A.introduce B.summon C.trigger D.create
问题16：
A.still B.also C.otherwise D.nevertheless
问题17：
A.sure B.sick C.aware D.tired
问题18：
A.tolerate B.repel C.neglect D.notice
问题19：
A.available B.reliable C.identifiable D.suitable
问题20：
A.similar to B.such as C.along with D.aside from