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问题详情：

Why do we laugh? For years scientists have asked themselves this question. No animals laugh and smile – only human beings, so does laughing help us to live longer? The answer seems to be that smiling and laughing help us to become calm and less worried, preventing illness which are caused by stress. Laughing is really the best medicine.

Scientists in California asked a group of students to act five different feelings: happiness, sadness, anger, surprise and fear – just using their faces. Changes in their heart rate, blood pressure and body temperature were recorded. Most of the feelings they acted didn’t cause changes, but when the students smiled their heart rate and their blood pressure went down. That happened even though they were only acting! So even if we feel sad inside, smiling makes us feel better.

Now, what makes us laugh? The answer to the question is far from simple. Everyone likes jokes, but we don’t laugh at the same ones. It depends largely on how old we are. There’s a kind of jokes, for example, which children love but older people might not find funny.

31. In the passage, the writer seems to pay more attention to _______.

A. comparison between human beings and animals

B. how laughing and smiling are connected with health

C. the degree to which laughing and smiling are harmful

D. finding out if any research has been done about laughing

32. The passage tells us that smiling and laughing ______.

A. have nothing to do with heart rate

B. do no good when someone is sad

C. lower one’s blood pressure

D. lower the temperature of the body

33. In the first paragraph, the underlined word “stress” can be best explained as “_______”.

A. pressure and worry B. work and duties