Have you ever visited the Water Cube?Yes.When I served as a volunteerin Beijing,I it twice

选B.一般过去时表示在过去某个时间发生的动作或状态；而过去完成时表示在过去某一时间之前完成的动作.根据句意,我参观水立方两次均发生在我当志愿者期间,而不是之前,故选B.再比如：

Have you had your lunch?
Yes,I had it at 12:00.

  2.I got to the station at 8:00,but the train had left.
  3.I got to the station at 8:00,but the train left at 7:30.
 4.By the end of last year,we had learned 1000 English words.
  5.When I arrived home, my parents had had dinner.
  6.When I arrived home, we had dinner together.