By the time he was twelve, Edison to make a living by himself.
A. would begin
B. has begun
C. had begun
D. began
网友回答
A. would begin
B. has begun
(答案→)C. had begun
D. began
解析：考查时态。By…到…为止；与完成时连用。后面接the time he was twelve是过去时，故主句中使用过去完成时。句意：到他12岁的时候，Edison已经开始靠自己谋生了。故C正确。