Why do we laugh?For years scientist have asked 21 this question. No other animals laugh

Why do we laugh？For years scientist have asked 21 this question. No other animals laugh and smile --- only human beings. So does laughter help us to survive in any way？The answer seems to be that smiling and laughter help us to 22 ，reducing tension and preventing illnesses which are caused by stress. Laughter really is the best medicine？Scientists in California asked a group of actors to act out five different feelings：happiness，sadness，anger，surprise and 23 --- just using their faces. Changes in their heart rate，blood pressure and temperature were written down. Most of the feelings they acted out didn’t cause changes，but when the actors smiles，their heart rate and their blood pressure 24 . This happened even though they were only acting!So even if we feel sad inside，smiling seems to make us feel better.So if you find yourself in a stressful situation，try to imagine how it must seemto an outsider 25 a sense of humor and let yourself laugh. (171 words)

21. A.relax

B.went down

C.fear

D.themselves

E. with

22. A.relax

B.went down

C.fear

D.themselves

E. with

23. A. relax

B. went down

C. fear

D. themselves

E. with

24. A.relax

B. went down

C. fear

D. themselves

E. with

25.A. relax

B. went down

C. fear

D. themselves

E. with