为啥选D,是虚拟语气主句和从句时态的问题吗,请说详细一点,

虚拟语气的问题
在非真实条件状语从句中
　　1.表示与现在事实相反的情况
　　If I were ten years younger,I would travel round the world.如果我年轻十岁,我就去环球旅行.
　　2.表示与过去事实相反的情况
　　If the traffic hadn’t been so heavy,I could have been back by seven o’clock.如果交通没有那么堵的话,在七点钟以前我就回来了.
　　3.表示与将来事实相反的情况
　　If it rained / were to rain / should rain tomorrow,the crops would be saved.如果明天下雨,庄稼就得救了.(最近天气很好,明天根本不会下雨)
再问： A不是省略 if 吗，为啥不选A
再答： 如果是省去的话，应该这样改Had it not been for ,..... 另附省略的例句： Were I you, I would never do that. 如果我是你，我永远不会做那件事。 Should I have time, I would call on her. 我要是有时间就去探望她。 Had you informed me earlier, I wouldn’t have signed the contract. 要是你早些告诉我的话，我就不会签那份合同了。