题目

It seems hard to watch someone yawn（打哈欠） and not to yawn ourselves. Even reading about yawning can make you do it. Now, a new study has found why yawning has such a powerful force.     Yawning when others yawn, the study suggests, is a sign of pity and a form of social connection. Kids don’t develop this deeply rooted behavior until around age four, the study found. Kids with autism (自闭症) are less likely to catch yawns. In the most serious cases, they never do. Yawning might help doctors to see whether the children are developing rightly. The work could also lead t18．一个“18V　6W”的小灯泡，如果接在36V电源上，为使其正常发光，需串联一个54Ω的电阻，该电阻消耗的功率是6W．