vvt发动机:______，many problems of space flight couldn’t have been solved

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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
虚拟时态

A if后面的是与现在事实相反的假设，所以用虚拟语气（时态倒退）

______，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