He is one of the world-famous musicians. By the time he was six, he posing.
A. had begun
B. begins
C. would begin
D. was beginning
网友回答
(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