听力原文：M: Congratulations on breaking the records at the recent Olympics, Miss Green !
W: Thank you, Mr Parker.
M: Oh, look at those trophies! You became a swimming star at the age of fifteen, didn't you？
W: Yeah, you're right.
M: Could you tell me something about your plans for the future？
W: To tell you the truth, I'm going to give up swimming from now on.
M: I beg your pardon？
W: I mean I'd rather not swim at all. I'm too old to. . .
M: Excuse me, if I'm not wrong, you're only twenty now. Is it too old twenty？
W: Yes, too old for a swimmer. If I swim in an international competition, I' ll hardly be able to win.
M: But. . . er. . . don't you enjoy swimming？
W: Certainly, and I enjoy visiting other countries, and the Olympics are very exciting. However, I missed more important things. I had to work very hard at swimming. There was training before school, after school, and at weekends. While other girls were growing up, I was swimming, swimming. But life isn't merely swimming, is it？
When did Miss Green become a swimming star？

At the age of 15.
B. At the age of 20.
C. At the age of 18.
D. At the age of 16.