He traveled to many mountain villages and saw many poor children out of school. This experience _______ his life.

A. would change
B. had changed
C. was to change
D. was changing

我认为，此题只能选择一个最佳答案，而无法选一个正确答案。除了had changed 不可以外，其余3 个都正确。理由：

A. would change （说话者的推测，would 表示“可能性”）
B. had changed（排除此答案，不可能早于前句发生）
C. was to change（过去将来时的一种。表示推测“注定要”）
D. was changing（traveled 和saw 这两个动作，是个反复的过程，不是瞬间结束。所以，was changing 表示，在过去某个时候就开始在改变着他的生活。）