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Do you laugh every day? Most people do．  1 That is a lot of laughter．

In India，there are hundreds of laughter clubs．  2 First they put up their hands above their heads．Then they pretend to laugh．Soon everyone is laughing happily．People say they feel good after laughing together．

Scientists believe that laughter is good for you．Why? For one thing，laughter is good exercise．  3 Scientists say that one hundred laughs equals ten minutes of running．Laughing helps you relax．That’s good for you，too．

Why do we laugh? That is a hard question to answer．  4 They don’t laugh very often when they are alone．Many scientists believe that we use laughter to get on well with other people．Laughter helps us feel part of a group．

5 Some think that laughter makes sick people get better．Do you think so，too?

下列事例中，属于内能转化为机械能的是[ ]

A、两手相互搓一搓，手心变热

B、用气筒给车打气时，气筒发热

C、水壶中的水沸腾时，壶盖被水蒸气顶起

D、流星在空中滑落时发光发热