He is one of the world-famous musicians. By the time he was six he composing.A. had begu

问题补充：

He is one of the world-famous musicians. By the time he was six, he composing.

A. had begun

B. begins

C. would begin

D. was beginning

答案：

(答案→)A. had begun

B. begins

C. would begin

D. was beginning

解析：考查谓语动词的时态 by +过去时间，谓语动词（主句）常用过去完成时态；by+将来时间，谓语动词（主句）常用将来完成时态