By the time he was twelve, Edison _______ to make a living by himself.
A.would begin
B.has begun
C.had begun
D.began
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C解析试题分析：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。考点：考查时态点评：时态题中对by的考查是最关键的，by到…时为止；与完成时连用。