单项选择题

Laughter
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 investigate laughter and the effects 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 beneficial.
Other tests have shown that laughter appears to be capable of reducing the effect of pain on the body. In one experiment doctors produced pain in groups of students who listened to different radio programs. The group which tolerated the pain for the longest time was the group which listened to a funny program. The reason why laughter can reduce pain seems to be that it helps to produce endorphins(内啡肽) in the brain. These are natural chemicals which diminish both stress and pain.
There is also some evidence to suggest that laughter helps the body’s immune system, that is the system which fights infection. 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 beneficial effects similar to those caused by laughter. We learn from the first paragraph that laughter ______.