--Havent you seen Mr. Wilson at the lecture?--Yes but he had left I could ask him any q

问题补充：

--Haven you seen Mr. Wilson at the lecture?

--Yes, but he had left I could ask him any questions.

A. when

B. before

C. after

D. as

答案：

A. when

(答案→)B. before

C. after

D. as

解析：此题考察的是过去完成时，had +动词的过去分词，表示发生在过去的过去的事情。