如何判断虚拟语气中的时态?（请看例题）

如何判断虚拟语气中的时态?（请看例题）
1、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
2、If it had not rained last, the road ____ so slippery now.
(A) would not be being
(B) would not have been
(C) would not be
(D) must not be
第1题答案：(A)
第2题答案：(C)
问题：
两道题中都有提示时间的词“now”,为什么第1题用“would have done”（过去）形式,第2题用“would do”（现在）形式呢?
解析说第1题是过去时态的虚拟语气,第2题是错综时态的虚拟语气.那么应如何判断呢?