单项选择题

Nose has it pretty hard, Boxers flatten them. Doctors rearrange them. People make jokes about their unflattering characteristics. Worst of all, when it comes to smell, no one really understands them.
Despite the nose’s conspicuous presence, its workings are subtle. Smell, or olfaction, is a chemo-sense, relying on specialized interactions between chemicals and nerve endings. When a rose, for example, is sniffed, odor molecules are carried by the rising air-stream to the top of the nasal cavity, just behind the bridge of the nose, where the tips of the tends of millions of olfactory nerve cells are clustered in the mucous lining. The molecules somehow trigger the nerve ending, white carry the message to the olfactory lobes of the brain. Because smell information then travels to other region of the brain, the scent of a rose can elicit not only a pleasure sensation but emotions and memories as well.
Though just how odors stimulate the nerves is unknown, scientists do know that our sense of smell is surprisingly keen capable of distinguishing up to tens of thousands of chemical odors. The laboratory task of isolating the components must of an odor is far from simple .Tobacco smoke, for example, is made up of several thousand different chemicals. Moreover smell by their sources or associations. Description such as "like a wet dog" or "like my elementary school" may convey perceptions but are vastly inadequate for labeling the chemistry involved.
To further complicate research, olfaction is connected to other sensations. Besides olfactory nerves, the nasal cavity contains pain-sensitive nerves that perceive sensations such as the kick in ammonia or the burning in chili peppers. Smell also inter-wines with taste to create flavor. A coffee drinker holding his nose while sipping would taste only the bitter in his brew, for taste receptors generally appear limited to bitter, salty, sour, and sweet. The sense of smell is ten thousand times more sensitive than taste and makes subtle distinctions among lemon, chocolate, and many more flavors.
So how does the nose manage this sophisticated discrimination Lack of evidence hasn’t kept scientists from speculating. One idea is that every odor molecule vibrates at its own frequency, creating patterns of disturbance in the air similar to the wave patterns produced by sound. According to this theory, the nerves act as receives for the unique vibrations of every odor molecule. The scheme requires no direct contact between the molecule and the nerve cell.
Another suggestion is that primary odors, equivalent to the primary colors of vision, underlie all smells and are detected by receptor sites on the olfactory nerves. Different combinations of about thirty basic smells, with labels such as malty, minty, and musky, could form an infinite number of odors.
Other scientists think that each smell is its own primary smell. They believe the olfactory nerve endings have specific receptor proteins that bind to each of the chemicals people can sense. This theory, however, calls for thousands of different proteins-none of which has been found.
"The science of smell is so empirical," says Robert Gesteland, a neurobiologist at Northwestern University, "there is no predictive base for experiments." Unlike the senses of sight, touch, and hearing, olfaction studies have attracted only a small share of scientific interest. That may change. Researchers hope that unraveling the mystery of smell will advance our understanding of the future, with enough known about smell, it might be possible to endow strange, unappealing but nutritious foods with more familiar odors, perhaps expanding the world’s food supply. For the moment, however, what the nose knows it isn’t revealing. The broadest example of a major problem facing smell researchers is contained in______.