【题目】Much labor would have been saved if the electronic computers_____________ before.
A. had invented B. were invented
C. should have been invented D. had been invented
在线课程【答案】D
【解析】
试题分析：考查虚拟语气。句意：如果以前电脑被发明出来，更多劳动力就会被节省下来。本句考查的是if引导的条件状语从句的虚拟语气，当与过去事实相反的时候，从句使用过去完成时，主句使用“情态动词+have+done”。根据句意可知本句使用被动语态，故D正确。