[填空题]The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, ________ this is largely because, ________ animals, we stand upright. This means that our noses are ________ to perceiving those smells which float through the air, ________ the majority of smells which stick to surfaces. In fact, ________ , we are extremely sensitive to smells, ________ we do not generally realize it. Our noses are capable of ________ human smells even when these are________to far below one part in one million. 　　Strangely, some people find that they can smell one type of flower but not another, ________ others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate ________ smell receptors in the nose. These receptors are the cells which sense smells and send ________ to the brain. However, it has been found that even people insensitive to a certain smell ________
A. A．when
B． since
C． for
D．whereas