单选题 To conduct some forms of sleep research, we have to find a way to track sleepiness over the day. Some people might believe that measuring sleepiness is a fairly trivial task. Couldn't you, for instance, simply count the number of times a person yawns during any given hour or so? In most people's minds, yawning—that slow, exaggerated mouth opening with the long, deep inhalation of air, followed by a briefer exhalation—is the most obvious sign of sleepiness. It is a common behavior shared by many animals, including our pet dogs and cats but also crocodiles, snakes, birds, and even some fishes. It is certainly true that sleepy people tend to yawn more than wide-awake people. It is also true that people who say they are bored by what is happening at the moment will tend to yawn more frequently. However, whether yawning is a sign that you are getting ready for sleep or that you are successfully fighting off sleep is not known. Simply stretching your body, as you might do if you have been siring in the same position for a long period of time, will often trigger a yawn. Unfortunately, yawns don't just indicate sleepiness. In some animals, yawning is a sign of stress. When a dog trainer sees a dog yawning in a dog obedience class, it is usually a sign that the animal is under a good deal of pressure. Perhaps the handler is pushing too hard or moving too fast for the dog to feel in control of the situation. A moment or two of play and then turning to another activity is usually enough to banish yawning for quite a while. Yawning can also be a sign of stress in humans. Once, when observing airborne troops about to take their first parachute jump, I noticed that several of the soldiers were sitting in the plane and yawning. It was 10 A. M. , just after a coffee break, and I doubted that they were tired; I knew for a fact that they were far too nervous to be bored. When I asked about this, the officer in charge laughed and said it was really quite a common behavior, especially on the first jump. There is also a social aspect to yawning. Psychologists have placed actors in crowded rooms and auditoriums and had them deliberately yawn. Within moments, there is usually an increase in yawning by everyone else in the room. Similarly, people who watch films or videos of others yawning are more likely to yawn. Even just reading about yawning tends to stimulate people to yawn. The truth of the matter is that we really don't know what purpose yawning serves. Scientists originally thought that the purpose of yawning was to increase the amount of oxygen in the blood or to release some accumulated carbon dioxide. We now know that this is not true, since increasing the concentration of carbon dioxide in the air seems not to make people more likely to yawn but to make them breathe faster to try to bring in more oxygen. On the other hand, breathing 100 percent pure oxygen does not seem to reduce the likelihood of yawning. Since yawning seems to be associated with a lot more than the need for sleep, we obviously have to find some odier measure of sleepiness. Some researchers have simply tried to ask people how sleepy they feel at any time using some sort of self-rating scale. There are, however, problems with getting people to make these types of judgments. Sometimes people simply lie to the researchers when asked about how sleepy they are. This occurs because in many areas of society admitting that one is fatigued and sleepy is considered a mark of weakness or lack of ambition and drive. In odier instances, people may admit they need four cups of coffee to make it through the morning, but it may never occur to them that this might be due to the fact that they are so sleepy that they need stimulation from caffeine to be able to do their required tasks. For these reasons, many researchers have developed an alternate method to determine how sleepy a person is. It is based upon a simple definition of sleep need: The greater your sleep need, or the sleepier you are, the faster you will fall asleep if given the opportunity to do so.

单选题 The question in lines 3-4(Paragraph 1)is based on which of the following assumptions?

单选题 The author uses which of the following in the fourth paragraph?

单选题 The author mentions the "coffee break" to emphasize that a______.

单选题 Which of the following, if true, would most directly disprove what "scientists originally thought"?

单选题 The passage as a whole is best characterized as______.