问题
选择题
By the time he was twelve, Edison ________ to make a living by himself .
A．would begin
B．has begun
C．had begun
D．was begun
答案
答案：C
时态题。By the time +过去时，主句谓语用过去完成时态 选C。 句意：到爱迪生十二岁时他已开始自己谋生了。
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