听力原文：W: What are you working now？
M: (19)I've just finished a piece of background music.
W: Background music？
M: Yes. (21)It’s supposed to influence your attitude, put you in the right mood.
W: I'm not sure I like that idea.
M: It seems to work. Factories use it a lot. (20) It makes the workers happy and work better that way. In one factory, music increased production 4.5%.
W: I think they'd get tired of hearing music all day.
M: They don't. If the music stops, somebody always runs to the telephone to complain.
W: I can't remember when there wasn't background music.
M: Actually, it started during WWII when some factories had their own orchestras to keep workers happy and calm. Now the music is piped in by a machine, and different kinds of music are played at different times during the day. They play faster music at ten in the morning than at eight, because workers tend to be slower then.
W: What about restaurants？ Do they play the same music for dinner or lunch？
M: I don't know. But I know hamburger places play fast music because they found a customer spent only seventeen minutes eating. The time was twenty-two minutes before that.
W: (22)80 they have more people coming in and out to buy hamburgers.
M: Exactly. (21)And that's good for business. You can see why music has become so popular.
W: There's still something about it that I don't quite like.
M: I know what you mean, but lots of people would not agree with you. Companies pay millions of dollars every year for background music. Prisons use it, and farmers use it to keep their cattle calm. It's even supposed to have an effect on plants.
(23)

A player of background music.
B. A researcher on the function of background music.
C. A composer of background music.
D. A fan of background music.