A、The sparkling of eyes. B、The opening of mouth. C、The movement of muscles. D、The blinking of eyes. A录音提到达尔文和他的一名同事证实，人们很容易就能判断一

An ancient Greek philosopher once wrote that laughter is what makes us human—that it defines us as a species. Much more recent developments in biology and behavioral science suggest that not only humans but also rats and dolphins laugh: nonetheless, laughter is one of the most important aspects of human social life and self-expression. Laughter starts very soon after an infant is born—almost as soon as crying—and it serves many different social and psychological functions, from sharing our joy to intimidating and insulting other people. In many parts of the world, making other people laugh is considered a great gift, and comedy has always been a vital part of culture and art—perhaps precisely because of the complex, significant role of laughter in everyday life.
    Humans begin laughing when they are about forty days old: in the 19th century, Charles Darwin verified this number by observing his own newborn daughter. Darwin also suggested an explanation for the sounds she made: we want to show one another that we are happy or delighted. Pleasure lies at the base of what Darwin called "real" laughter: most people still think of laughing this way, as a fundamentally social act that helps us relate to one another by communicating positive feelings. It’s an easily verifiable fact that everyone laughs louder and more frequently in groups than when alone. And Darwin and one of his colleagues also demonstrated that it is easy for human beings to tell when someone is faking a laugh or a smile. Because of a certain muscle in the human face—the zygomaticus major, or muscle of joy—our eyes sparkle when we are genuinely happy.
    Comedy and laughter have played and still play a vital role in artistic production in western culture. Comedy as we know it—that is, staged performances intended to induce laughter in viewers or audience members—was born in ancient Greece. During the next several centuries, Italy, England, France and Germany all developed strong comic traditions—especially in Italy, where a theater tradition called the commedia dell’arte was born. Traveling performers played comic songs and put on puppet shows and plays to amuse crowds of viewers. This continued across Europe for hundreds of years, even as major cities developed and acting troupes settled into more permanent theaters. Even the American version of the television stand-up comedian is also very old, and can be traced back to the court fool or jester in the middle ages and the Renaissance. Human beings may not be the only animals who laugh—but for centuries we have made it a central part of being human.
16 What do we learn about laughter?
17 What indicates a real laughter according to Darwin?
18 What do we learn about comedy?