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 ________ can suddenly become sensitive to it when ________ to it often enough. The explanation for insensitivity to smell seems to be that the brain finds it ________ to keep all smell receptors working all the time but can ________ new receptors if necessary. This may ________ explain why we are not usually sensitive to our own smells―we simply do not need to be. We are not ________ of the usual smell of our own house, but we ________ new smells when we visit someone else’ s. The brain finds it best to keep smell receptors ________ for unfamiliar and emergency signals ________ the smell of smoke, which might indicate the danger of fire.