Decide which of the words given in the box below would best complete the passage if inserted in the corresponding blank. The words can be used ONCE ONLY. Mark the letter for each word on ANSWER SHEET. 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 card never inquired into the identity of the ___31___ professor. They received his holiday greeting card, and they ___32___ 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 ___33___, what another person has provided us. If a woman does us a favor, we should do her ___34___ in return; if a man sends us a birthday present, we should remember his birthday with a gift of our own; if a ___35___ invites us to a party, we should be sure to invite them to one of ours. By ___36___ of the reciprocity rule, then, we are obligated to the future repayment of favors, gifts, invitations, and the like. So typical is it for ___37___ 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 ___38___ with it is its pervasiveness in human culture. It is so ___39___ that after intensive study, sociologists such as Alvin Goldener can report that there is no human society that does not ___40___ to the rule.