教师招聘条件:______,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)