By the time hefourteen years old, headvanced mathematics all by hims

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问题 By the time he______fourteen years old, he______advanced mathematics all by himself.

选项 A、has been: learned
B、had been: had learned
C、was: learned
D、was: had learned

答案 D

解析本题考查动词的时态。“by+过去时间”是过去完成时的时态标志，因此本句主句部分谓语动词用过去完成时，而从句部分谓语动词用一般过去时。D为正确选项。

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By the time he______fourteen years old, he______advanced mathematics all by hims