One of the most important features that distinguish man from animals is the ability to laugh. People who have investigated the phenomenon have offered many theories to explain human laughter. Scholars in the field of psychology, for example, have done 42 on the subject of what makes people laugh. But, as usually happens, the experts disagree, and there is much 43 in the field. Some people claim that human beings laugh at things which are strange to their experience. Others feel that people laugh at what they secretly believe to be their own weaknesses. It is 44 that all people do not laugh at the same things. Sometimes people laugh at what they see; sometimes they laugh at what they hear. Humor often 45 on a knowledge of certain words or even on an understanding of a particular cultural background. There are, of course, many different sources of humor, but the important fact is that all people share the great pleasure of laughter. 【題組】45 (A)depends (B)keeps (C)looks (D)stays