would和should的用法,虚拟语气中有什么区别?

would和should的用法,虚拟语气中有什么区别?
以括号中所给词的适当形式填空：
The petrol tank is empty.I ______(fill) it up before we left home.
答案是should have filled
请问1、这道题是虚拟语气吗?
2、为什么不能用“would have filled”?
关于这个问题的第2个例题：
Had electronic computers not being invented,many problems of space flight _____ now.
(A) would not have been solved
(B) would not be solved
(C) must not be solved
(D) should not have been solved
麻烦请问B、C、D为什么不对？