问题
选择题
—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
答案
答案：B
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。