Question

【题目】If the new safety system _______ to use, the accident would never have happened.

A. had been put B. were put C. should be put D. would be put

Answer 1

【答案】A

【解析】试题分析：句意：如果这个新的安全系统被投入使用过的话，这个事故就不会发生了。根据主句确定是对过去情况的虚拟，所以从句谓语用had done形式，此外根据句意可知是被动，用被动语态，故A项是正确的。

	if从句的谓语动词	主句谓语动词
1.与现在事实相反	动词过去式（be一律用were）	would/could/might/should+动词原形
2.与过去事实相反	had done	would/could/might/should+have done
3.与将来事实可能相反	a.动词过去式（be一律用were） b.should+动词原形（不能用would） c.were to+动词原形	would/could/might/should+动词原形