单选题：The human nose is an underrated tool. Humans are often thoug

The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals,51this is largely, because, unlike animals, we stand upright. Thismeans that our noses are52to perceiving those smells which float through the air, missing themajority of smells which stick to surfaces. In fact,53, we are extremely sensitive to smells,even if we do not generally realize it. Our noses are capable of54human smells even whenthese are diluted to far below one part in one million.
Strangely, some people find that they can smell one type of flower but not another,55others are sensitive to the smells of both flowers. This may be because some people do not have thegenes necessary to generate particular smell receptors in the nose. These receptors are the cellswhich sense smells and send56to the brain. However, it has been found that even peopleinsensitive to a certain smell at first can suddenly become sensitive to it when57to it oftenenough.
The explanation for insensitivity to smell seems to be that the brain finds it inefficient to keepall smell receptors working all the time but can58new receptors if necessary. This may alsoexplain why we are not usually sensitive to our own smells--we simply do not need to be. We arenot59of the usual smell of our own house, but we notice new smells when we visit someoneelse's. The brain finds it's best to keep smell receptors60for unfamiliar and emergency signalssuch as the smell of smoke, which might indicate the danger of fire.
阅读以上短文，回答题。

第51题答案是_________ A．although
B．as
C．but
D．while