请阅读Passage 2，完成第1—5小题。

Passage 2

   Anthropologists believe that laughter dates back millions of year--we were laughing long before we were verbally communicating. Whether you speak Chinese,English or Swahili,a laugh or smilc has virtually Lhe same meaning all over the world.

   Human beings love to laugh,and Lhe average adult laughs 17 times a day. Humans love to laugh so much that there are actually industries built around laughter. Jokes,situation comedies and comedi-.ans are all designed to get us laughing,because laughing makes us feel good. But why do we laugh?After all,iCs not exactly a trait we share with other animal species. Philosopher John Morreall believes that the first human laughter may have begun as a gesture of shared relief at the pasqing of danger.And qince the relaxation that results from laughter inhibits the biological fight-or-light response,laughter may indicate trust in one's companions.

   Many researchers believe that the purpose of laughter is related co making and strengthening human connections. Laughter occurs when people are comfortable with one another,when they feel open and free. And the more laughter there is,the more bonding occurs wiLhin the group. This feedback "loop" of bonding-laughter-more bonding,combined with the common desire not to be singled out from the group,may be another reason why laughter is often contagious. We laugh 30 times as much when we're with oLher people than we do when we are alone and laughing is not a solo activity.

   Laughter is a great thing-that's why we've all heard the saying, " Laughter is the best medicine. " There is strong evidence that laughter can actually do a number of positive physical things to improve health and help fight diseases. Blood pressure is lowered,and there is an increase in blood flow and in oxygenation of the blood,which further assists healing. In addition,people often store negative emotions,such as anger,sadness and fear,rather than expressing them. Laughter provides a way for these emotions to be harmlessly released. That's why s"me people who are upset or stressed out go to a funny movie or a comedy c:lub,so they can laugh the negative emotions away. Otherwise,theae negative emotions,when held inside,may cause biochemical changes that can affect our bodies.What may surprise you even more is the fact that researchers estimate that laughing 100 times is equal to 10 minutes on the rowing machine or 15 minutes on an exercise bike. lAughing can be a total body workout !