Read the following text. Choose the best word（s） for each numbered blank and mark A, B, C or D on ANSWER SHEET 1（10 points）
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.