He is the teacher by whom I ________ taught.
He is the teacher by whom I ________ taught.
A.have once been
B.had once been
C.have once
D.had once
答案及解析： A
解析：
导解：teach动作发生在过去，且与主语I为动宾关系．
He is the teacher by whom I ________ taught.
A.have once been
B.had once been
C.have once
D.had once
答案及解析： A
解析：
导解：teach动作发生在过去，且与主语I为动宾关系．