问题补充：
--? –I went to the mountains.
A. Where did you go on vacation?
B. What are you doing for vacation?
C. How did you go to the mountains?
D. How often do you go to the mountains?
答案：
(答案→)A. Where did you go on vacation?
B. What are you doing for vacation?
C. How did you go to the mountains?
D. How often do you go to the mountains?
解析：根据回答语我去爬山了，可知上文应问假期你去哪了，前后时态保持一致，都用过去时，故选A。
