C

    There is an old saying in English, "Laughter is the best medicine". Until recently, few people took the saying very seriously. Now however, doctors have begun to study laughter and the effects (作用) it has on the human body. They have found evidence that laughter really can improve people's health.

    Tests were carried out to study the effects of laughter on the body. People watched funny films while doctors checked their heart rate, blood pressure, breathing and muscles. It was found that laughter has similar effects to physical exercise. It increases blood pressure, the heart rate and the rate of breathing. It also works several groups of muscles in the face, the stomach and even the feet. If laughter exercises the body, it must be helpful.

    Other tests have shown that laughter appears to make the effect of pain on the body less. In one experiment doctors produced pain in groups of students who listened to different radio programs. The group which could bear the pain for the longest time was the group which listened to a funny program. The reason why laughter can make pain less seems to be that it helps to produce endorphins (内啡肽) in the brain. These are natural chemicals which make both stress and pain less.

    There is also some evidence to suggest that laughter helps the body's immune system that is the system which fights infection (感染). In an experiment, one group of students watched a funny video while another group served as the control group — in other words, a group with which to compare the first group. Doctors checked the blood of the students in both groups and found that the people in the group that watched the video had an increase in the activity of their white blood cells that is the cells which fight infection.

    As a result of these discoveries, some doctors and psychiatrists (精神病学) in the United States now hold laughter clinics, in which they try to improve their patients' condition by encouraging them to laugh. They have found that even if their patients do not really feel like laughing, making them smile is enough to produce good effects similar to those caused by laughter.

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一天半夜里，我突然被一阵高呼声惊醒，几位朝廷大臣要求我马上赶到皇宫去，原来是皇后的寝宫失火了。我一路小心赶往皇宫，没有踏伤一个行人。皇后寝宫火势很猛，虽然侍卫们一桶一桶地把水供应给我，但因水源离这儿很远，显然无济于事。看来这座富丽堂皇的宫殿免不了要烧成平地。幸亏我当时心眼特别活，忽然想起一条妙计：我在适当的地方撒了一大泡尿，所以不到三分钟火就全熄了，这才把他们费了多少年心血建造的宫殿救了下来。                                              

完成一个动作时，运动系统的____ 起着动力的作用，牵引着骨绕着____ 运动。

–David, is that _____ English teacher?

                         --Yes, he is ___. He is very caring.

宋元时期是我国古代科技与文学艺术发展的高峰时期，当时我国不仅科技水平处于世界领先地位，文学艺术也灿烂辉煌，在史学、文学、书法、绘画等领域都成就斐然，并涌现出一大批文人墨客。请结合所学知识，探究以下问题。