—Have you ever visited the Water Cube?
—Yes. When I served as a volunteer in Beijing, I it twice.
A. have visited
B. visited
C. had visited
D. had been visiting
网友回答
A. have visited
(答案→)B. visited
C. had visited
D. had been visiting
解析：考察时态。根据when I served as a volunteer,可知，该句主句用过去时，
句意为，---你去过水立方吗？--是的，当我作志愿者的时候，我去过两次。