【正确答案】 D

【答案解析】[听力原文]
Every human being, no matter where they are from, is born with the ability to smile. A smile is a natural reaction to a positive moment, like a friendly face, or a clever joke. Most babies first smile between six and eight weeks old. At first, it is only an expression made when excursing their facial muscles or passing wind. But once they realize a smile gets them a lot of attention, huge smiles in return, happy noises, extra treats, they learn to try it again. And why wouldn't they? It takes forty-three muscles to frown, but only seventeen to smile. We smile when we are happy. We smile when we see people we know. But what happens when you are not happy to see someone you know? You smile any way. You fake a smile. Unfortunately, however, a fake smile never looks quite the same as the real one. In a genuine smile, you not only show your lower teeth, but also move the muscles running all the way around the mouth which in turn makes the skin around the eyes become tighter. And then your smile is real. However, when someone smiles politely, rather than because they really want to, they use the muscle, which only raises the size of the mouth, and which does not move the muscles at the corner of the eyes. This kind of fake smile is sometimes refer to as the "Pan-Am Smile". As it is named, after the former airline whose flight attendants welcomed every passenger with the same force smile.

According to the talk, what is a smile?