A. accompanies B. automatically C. couple D. goes E.indebtedness F.kind G.manner H. mercifully I. one J. obey K. renowned L. subscribe M. unknown N. virtue O.widespread A few years ago, a university professor tried a little experiment. He sent Christmas cards to a sample of perfect strangers. Although he expected some reaction, the response he received was amazing-holiday cards addressed to him came pouring back from the people who had never met nor heard of him. The great majority of those who returned a car never inquired into the identity of the_____(1)professor. They received his holiday greeting cards, and they _____ (2) sent one in return. This study shows the action of one of the most powerful of the weapons of influence around us--the rule for reciprocation. The rule says that we should try to repay, in ____(3) , what another person has provided us. If a woman does us a favor, we should do her ____ (4) in return; if a man sends us a birthday present, we should remember his birthday with a gift of our own; if a ____(5) invites us to a party, we should be sure to invite them to one of ours. By _____(6) of the reciprocity rule, then, we are obliged to the future repayment of favors, gifts, invitations, and the like. So typical is it for ____(7) to accompany the receipt of such things that a term like “ much obliged ” has become a synonym for “ thank you ” , not only in the English language but in others as well. The impressive aspect of the rule for reciprocation and the sense of obligation that ____(8) with it is its pervasiveness in human culture. It is so _____(9) that after intensive study, sociologists can report that there is no human society that does not _____(10) to the rule.