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The Mystery of Yawning According to conventional theory, yawning takes place when people are bored or sleepy and serves the function of increasing alertness by reversing, through deeper breathing, the drop in blood oxygen levels that are caused by the shallow breathing that accompanies lack of sleep or boredom. Unfortunately, the few scientific investigations of yawning have failed to find any connection between how often someone yawns and how much sleep they have had or how tired they are. About the closest any research has come to supporting the tiredness theory is to confirm that adults yawn more often on weekdays than at weekends, and that school children yawn more frequently in their first year at primary school than they do in kindergarten. Another flaw of the tiredness theory is that yawning does not raise alertness or physiological activity, as the theory would predict. When researchers measured the heart rate, muscle tension and skin conductance of people before, during and after yawning, they did detect some changes in skin conductance following yawning, indicating a slight increase in physiological activity. However, similar changes occurred when the subjects were asked simply to open their mouths or to breathe deeply. Yawning did nothing special to their state of physiological activity. Experiments have also cast serious doubt on the belief that yawning is triggered by a drop in blood oxygen or a rise in blood carbon dioxide. Volunteers were told to think about yawning while they breathed either normal air, pure oxygen, or an air mixture with an above-normal level of carbon dioxide. If the theory was correct, breathing air with extra carbon dioxide should have triggered yawning, while breathing pure oxygen should have suppressed yawning. In fact, neither condition made any difference to the frequency of yawning, which remained constant at about 24 yawns per hour. Another experiment demonstrated that physical exercise, which was sufficiently vigorous to double the rate of breathing, had no effect on the frequency of yawning. Again the implication is that yawning has little or nothing to do with oxygen. The word "flaw" in the passage is closest in meaning to （）.
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Many people wrongly believe that when people reach old age, their families place them in nursing homes. They are left in the 1 of strangers for the rest of their lives. Their growing children visit them only occasionally, but more often, they do not have any 2 visitors. The truth is that this idea is an unfortunate myth and imaginary story. In fact, family members provide over 80 percent of the care that elderly people need. Samuel Preston, a sociologist, studied 3 the American family is changing. He reported that by the time the average American couple reaches 40 years of age, they have more parents than children. 4 , because people today live longer after an illness than people did years 5 , family members must provide long term care. More psychologists have found that all caregivers share a common characteristic: all caregivers believe that they are the best 6 for the job. In other words, they all felt that they 7 do the job better than anyone else. Social workers 8 caregivers to find out why they took on the responsibility of caring for an elderly relative. Many caregivers believed they had 9 to help their relative. Some stated that helping others made them feel more useful. Others hoped that by helping someone now, they would deserve care when they became old and 10 . Caring for the elderly and being taken care of can be a mutually satisfying experience for everyone who might be involved. （4）处填入（）
Many people wrongly believe that when people reach old age, their families place them in nursing homes. They are left in the 1 of strangers for the rest of their lives. Their growing children visit them only occasionally, but more often, they do not have any 2 visitors. The truth is that this idea is an unfortunate myth and imaginary story. In fact, family members provide over 80 percent of the care that elderly people need. Samuel Preston, a sociologist, studied 3 the American family is changing. He reported that by the time the average American couple reaches 40 years of age, they have more parents than children. 4 , because people today live longer after an illness than people did years 5 , family members must provide long term care. More psychologists have found that all caregivers share a common characteristic: all caregivers believe that they are the best 6 for the job. In other words, they all felt that they 7 do the job better than anyone else. Social workers 8 caregivers to find out why they took on the responsibility of caring for an elderly relative. Many caregivers believed they had 9 to help their relative. Some stated that helping others made them feel more useful. Others hoped that by helping someone now, they would deserve care when they became old and 10 . Caring for the elderly and being taken care of can be a mutually satisfying experience for everyone who might be involved. （7）处填入（）
Many people wrongly believe that when people reach old age, their families place them in nursing homes. They are left in the 1 of strangers for the rest of their lives. Their growing children visit them only occasionally, but more often, they do not have any 2 visitors. The truth is that this idea is an unfortunate myth and imaginary story. In fact, family members provide over 80 percent of the care that elderly people need. Samuel Preston, a sociologist, studied 3 the American family is changing. He reported that by the time the average American couple reaches 40 years of age, they have more parents than children. 4 , because people today live longer after an illness than people did years 5 , family members must provide long term care. More psychologists have found that all caregivers share a common characteristic: all caregivers believe that they are the best 6 for the job. In other words, they all felt that they 7 do the job better than anyone else. Social workers 8 caregivers to find out why they took on the responsibility of caring for an elderly relative. Many caregivers believed they had 9 to help their relative. Some stated that helping others made them feel more useful. Others hoped that by helping someone now, they would deserve care when they became old and 10 . Caring for the elderly and being taken care of can be a mutually satisfying experience for everyone who might be involved. （3）处填入（）
The Mystery of Yawning According to conventional theory, yawning takes place when people are bored or sleepy and serves the function of increasing alertness by reversing, through deeper breathing, the drop in blood oxygen levels that are caused by the shallow breathing that accompanies lack of sleep or boredom. Unfortunately, the few scientific investigations of yawning have failed to find any connection between how often someone yawns and how much sleep they have had or how tired they are. About the closest any research has come to supporting the tiredness theory is to confirm that adults yawn more often on weekdays than at weekends, and that school children yawn more frequently in their first year at primary school than they do in kindergarten. Another flaw of the tiredness theory is that yawning does not raise alertness or physiological activity, as the theory would predict. When researchers measured the heart rate, muscle tension and skin conductance of people before, during and after yawning, they did detect some changes in skin conductance following yawning, indicating a slight increase in physiological activity. However, similar changes occurred when the subjects were asked simply to open their mouths or to breathe deeply. Yawning did nothing special to their state of physiological activity. Experiments have also cast serious doubt on the belief that yawning is triggered by a drop in blood oxygen or a rise in blood carbon dioxide. Volunteers were told to think about yawning while they breathed either normal air, pure oxygen, or an air mixture with an above-normal level of carbon dioxide. If the theory was correct, breathing air with extra carbon dioxide should have triggered yawning, while breathing pure oxygen should have suppressed yawning. In fact, neither condition made any difference to the frequency of yawning, which remained constant at about 24 yawns per hour. Another experiment demonstrated that physical exercise, which was sufficiently vigorous to double the rate of breathing, had no effect on the frequency of yawning. Again the implication is that yawning has little or nothing to do with oxygen. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage Incorrect choices change the meaning in important ways or leave out essential information.
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