Ancient Greek philosopher Aristotle viewed laughter as "a bodily exercise precious to health。" But 1 despite some claims to the contrary, chuckling probably has little influence on physical fitness. Laughter does 2 produce short-term changes in cardiovascular function and respiration, 3 boosting heart rate, respiratory rate and depth, as well as oxygen consumption. But because hard laughter is difficult to 4 sustain, a good guffaw is unlikely to have 5 measurable cardiovascular benefits the way, say, walking or jogging does。  6 In fact, instead of straining muscles to build them, as exercise does, laughter apparently accomplishes the 7 opposite. Studies dating back to the 1930s indicate that laughter 8 relaxes muscles, decreasing muscle tone for up to 45 minutes after the guffaw subsides。  Such physical relaxation might conceivably help 9 moderate the effects of psychological stress. After all, the act of laughing probably does produce other types of 10 physical feedback that improve an individual”s emotional state. 11 According to one classical theory of emotion, our feelings are partially rooted 12 in physical reactions. American  psychologist William James and Danish physiologist Carl Lange argued at the end of the 19th century that humans do not cry 13 because they are sad but that they become sad when the tears begin to flow。  Although sadness also 14 precedes tears, evidence suggests that emotions can flow 15 from muscular responses. In an experiment published in 1988, social psychologist Fritz Strack of the University of Würzburg in Germany and his colleagues asked volunteers to 16 hold a pen either with their teeth-thereby creating an artificial smile-or with their lips, which would produce a 17 disappointed expression. Those forced to exercise their smiling muscles 18 reacted more exuberantly to funny cartoons than did those whose mouths were contracted in a frown, 19 suggesting that  expressions may influence emotions rather than just the other way around. 20 Similarly, the physical act of laughter could improve mood。 1.[A]among [B]except [C]despite [D]like C 2.[A]reflect [B]demand [C]indicate [D]produce D