The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, but this is largely because, ___（1）___ animals, we stand upright. This means that our noses are limited to perceiving those smells that float through the air, ___（2）___ the majority of smells which stick to surfaces. In fact though, we are extremely sensitive to smells, ___（3）___ we do not generally realize it. Our noses are capable of detecting human smells even when these are ___（4）___ to far below one part in one million.
Strangely, some people find that they can smell one type of flower but not another, whereas others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate ___（5）___smell receptors in the nose. These receptors are the cells which sense smells and send messages to the brain. However, it has been found that even people insensitive to a certain smell ___（6）___ can suddenly become sensitive to it when exposed to it often enough.
The explanation for insensitivity to smell seems to be that brain finds it ___（7）___ to keep all smell receptors working all the time but can create new receptors if necessary. This may ___（8）___ 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 ___（9）___ new smells when we visit someone else's. The brain finds it best to keep smell receptors available for unfamiliar and emergency signals ___（10）___ the smell of smoke, which might indicate the danger of fire.
（8）下列正确的是（ ）

A.still

B.also

C.otherwise

D.nevertheless