A、High altitude. B、The cold weather. C、Mild exercises. D、The food. AAccording to the woman, what may cause the man the most prob

M: I have been tired of working and need a change. So I’m just making plans to go away during Spring Festival.
W: Really? Where are you going?
M: I’m planning to visit Tibet.
W: My brother and I had a vacation there last year and we had a great time.
M: Did you get into Lasa?
W: Sure. That’s the first stop. No one would miss Lasa.
M: Is the Potala Palace magnificent?
W: You bet. And don’t forget the Barkhor Street. That place is full of treasures, I mean, all the exotic paintings and handicrafts, and the hand-made jewelries.
M: Well, as them is so much there waiting for mc, I guess I should stay longer.
W: Definitely.
M: I heard that the weather there is great.
W: Well, that depends on how you define "great". No humidity, but it could be extremely cold in winter. Besides, you do need to be careful about high altitude.
M: What should I do about that?
W: Oh, just take it easy for a few days. Don’t exercise too vigorously. Just do everything gradually.
M: I’m sure I will be fine. And I will let you know all about my trip when I come back.