【托福阅读题型：The mystery of　yawning句子插入题】托福阅读中句子插入题是重要题型之一。根据托福阅读题型不同来进行专项练习，是短期提高托福阅读能力的有效方法。同学们需要多加练习哦!

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　　托福阅读句子插入题：The mystery of　yawning

　　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 slightincrease 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 mystery of　yawning)

　　Look at the four squares [■] that indicate where the following sentence could be added to the passage.

　　This, however, was not the case

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