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.
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