By the time Einstein was 14 years old, he ________ himself advanced mathematics.
题型：不详难度：来源：
By the time Einstein was 14 years old, he ________ himself advanced mathematics.| A．has taught | B．taught | | C．had taught | D．was teaching |
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答案
| C |
解析
| by the time 引导时间状语从句，主句中的动作发生在从句中的动作之前。故用过去完成时。 |
举一反三
We have done things we ought not to have done and ________ undone things we ought to have done.| A．left | B．leave | | C．will leave | D．leaving |
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— Why weren"t you at the meeting?
— I ________ for a long distance call from my sister in America.| A．was waited | B．was waiting | | C．has waited | D．have been waiting |
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The little boy is dirty from head to foot because he ________ in the mud all morning.| A．has played | B．is playing | | C．has been playing | D．was playing |
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The new dictionaries are very useful. They ________ well and ________ already .| A．sell; have been sold out | | B．sold; had sold out | | C．sell; sell out | | D．are sold; have been sold out |
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It seems water ______ from this tap for some time. We’ll have to take it apart to put it right. | A．had leaked | B．is leaking | | C．leaked | D．has been leaking |
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