A．Thin air. B．High longitude. C．High altitude. D．Too Cold.
参考答案C解析:According to the woman, what is the biggest problem of the man in Zermatt?
推断题。根据No humidity，moderate temperatures，but you do need to be careful about high altitude if you go up the mountains．可知并非寒冷或者湿度问题，是海拔太高，人不适应。因此，正确答案为C。