英国皇家警察学院:______，many problems of space flight couldn’t have been solved

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A. If electronic computers were not invented
B. If electronic computers are not invented
C. Had electronic computers not been invented
D. Had electronic computers been invented

C
=if electronic computers had not been invented
虚拟时

C
省略if的虚拟时态

______，many problems of space flight couldn’t have been solved ______，many problems of space flight couldn’t have been solved ______，many problems of space flight couldn’t have been solved 怎样翻译 many problems are routine or anticipated they can help peopleto w_____out many problems they can't do easily. English problems english problems ENGLISH PROBLEMS English Problems some problems