关于“引导虚拟语气的宾语从句的动词”引导的宾语从句里的虚拟条件句的里的动词形式应该是怎么样的呢？

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关于“引导虚拟语气的宾语从句的动词”引导的宾语从句里的虚拟条件句的一个问题。
关于宾语从句里的虚拟条件句，如果引导宾语从句的词是assume,mandate这种宾语从句里要用动词原形来表示虚拟语气的，那宾语从句里的主句，条件从句的动词分别应该是什么形式啊？
如果宾语从句里的主句还是用“动词原形”，那不就不符合虚拟条件句的格式（should + 动词原形 或者 should + have done）了吗？而且下面这个题目也不是用动词原形的，而是用would + do 的。
而且assumed是过去式，说明这是发生在过去，那么，宾语从句里的虚拟条件句应该是假设过去将来一个不是很可能发生的情况。额。。总之，关于“引导虚拟语气的宾语从句的动词”引导的宾语从句里的虚拟条件句的里的动词形式应该是怎么样的呢？
比方说这个题目：
Affording strategic proximity to the Strait of Gibraltar, Morocco was also of interest to the French throughout the first half of the twentieth century because they assumed that if they did not hold it, their grip on Algeria was always insecure
(A) if they did not hold it, their grip in Algeria was always insecure
(B) without it their grip on Algeria would never be secure
(C) their grip on Algeria was not ever secure if they did not hold it&H
(D) without that, they could never be secure about their grip on Algeria2o
(E) never would their grip on Algeria be secure if they did not hold it
正确:B  为什么呢？引导宾语从句的虚拟语气的动词后面又加上了if引导的虚拟语气，主句动词应该用什么形式？