A、Thin air problem. B、High longitude problem. C、High altitude problem. D、Cold problem. C推断题。根据原文No humidity，moderate temperature

M: I have been studying too much and need a change. So I’ve just made plans to go away during January holiday.
W: Really? Where are you going?
M: I’m planning to visit Switzerland.
W: My sister and I vacationed there last year and we had a great time.
M: Did you get into Zermatt?
W: Sure. Whenever we were skiing, we would be there. It’ s such a great place for skiing.
M: Is it far from the mountains?
W: Not at all. See even thought Zermatt is on a high flat plateau. There are even higher mountains near it. Just half an hour away from the city there are snow-covered slopes. You’ll have great fun just appreciating and hiking up the mountains.
M: Well. As the mountains are just thirty minutes away, I guess I should take my ice skate and my sled.
W: Definitely. You should really enjoy skating and skiing there. I heard you are really good at skiing.
M: I love it. If I do things out of passion, I’ll have it done very well. I heard that the weather there is great, isn’t it?
W: It is. No humidity, moderate temperatures, but you do need to be careful about high altitude if you go up the mountains. Some people may feel very uncomfortable if they are not used to it.
M: What should I do about that?
W: Oh, just take it easy for a few days. Don’ t go hiking up the mountains or exercise too much. Just do everything gradually until you feel comfortable for everything.
M: I ’m sure I will be fine. And I will let you know all about my trip when I come back.
Questions 9 to 11 are based on the conversation you have just heard.
9. What’ s the main purpose of the man’ s trip?
10. How did the woman know Zermatt?
11. According to the woman, what may cause the man the most problems in Zermatt ?