D. education, medicine and sports

　　参考答案1—4、BCAD

　　2016高考训练--阅读理解阅读下列短文，从每题所给的四个选项（A、B、C、D）中，选出最佳选项It is commonly believed that people yawn just when they are sleepy or tired as a result of their brain needing oxygen. However, a latest research shows that a yawn can help cool the brain and helps it work more effectively. Researchers said that raising or lowering oxygen and carbon dioxide levels in the blood did not produce that reaction.

　　Yawning involves opening the mouth while taking a long, deep breath of air. Researchers at the University of Albany in New York said those who held a cool pack to their forehead were less likely to yawn than those who held a warm or room-temperature pack while they were watching people yawning. They also found that those who breathed through the nose rather than the mouth were less likely to yawn. This was because the blood vessels（脉管）in the nasal cavity (腔) sent cool blood to the brain.

　　Since yawning occurs when brain temperature rises, sending cool blood to the brain serves to maintain the best levels of mental efficiency. This could help us to understand better the way our brains work. It may also help us understand the reason why frequent yawning can sometimes be a sign of brain disease.

　　25. According to the new study, people yawn when

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