A、It helps people get up early. B、It produces Vitamin D. C、It kills cold viruses. D、It enables us to look healthy. BB为两次提及的明示信息，

W: Good morning, class. Today I thought we would talk about something bright and cheerful—the sun. Can anyone tell me how you feel when it’s sunny compared to how you feel when it’s cloudy?
M: I find that it’s a lot easier to get up in the morning if it’s sunny.
W: Well, what if I told you that it would rain for weeks on end with no break.
M: I don’t think I would like that very much.
W: Most of the articles we read lately regarding the sun are discussing the negative effects of staying in the sun for too long, saying that ultraviolet radiation from the sun is a major cause of skin cancer. Although these articles are factual, the sun does have some positive side effects as well. As a matter of fact some scientists believe sunlight may reduce the risk of several types of cancer.
M: Excuse me, but many articles that I’ve read state that it can take as little as 30 minutes to get a sunburn.
W: True, but in small doses, the sun can be quite healthy for you. In fact, recent studies have found that sunlight can actually help protect you from certain types of cancer such as breast and stomach. More importantly, the sun also provides us with our main source of Vitamin D.
M: Well, I don’t need to worry about vitamins because I take multi-vitamins every day.
W: That’s great, but wouldn’t it be better if you didn’t have to take them and you were able to get vitamins in a more natural way such as food and the sun?
M: Does that mean we should spend all day in the sun without any protection?
W: No, spending roughly ten minutes per day in the sun should be enough to supply us with all the Vitamin D that we need.
19. What does the teacher mainly want to tell her students in this class?
20. What kind of cancer may the sunlight protects us from?
21. What is the main function of sunshine when concerned with our bodies?