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In the late seventh century, in a dispute over whether the Prophet Muhammad’s son-in-law, Ali, should carry on as the fourth caliph, Muhammad’s successor, Islam split into two branches, the Sunnis and the Shiites.

(A) over whether the Prophet Muhammad’s son-in-law, Ali, should carry on as the fourth caliph, Muhammad’s successor

(B) over if Ali, the Prophet Muhammad’s son-in-law, was going to carry on and be the fourth caliph, Muhammad’s successor

(C) over whether Ali, the Prophet Muhammad’s son-in-law, was going to carry on and be the fourth caliph, Muhammad’s successor

(D) as to whether the fourth caliph, Muhammad’s successor, is to be the Prophet Muhammad’s son-in-low, Ali

(E) concerning if the fourth caliph, Muhammad’s successor, was to be the Prophet Muhammad’s son-in-law, Ali

剩下a和c怎么选择正确项，我觉得 the Prophet Muhammad’s son-in-law做Ali的同位语，比a的表达好像好些呀？而且a为什么要用should? 想不明白，请指教，谢谢！！！

the Prophet Muhammad’s son-in-law做Ali的同位语——这是effectiveness的问题，不是correctness的问题，

c有两个地方我认为不妥，一个是be going to ,这里显得不通顺;用should，正好说明讨论XX是否应该继位，

一个是carry on and be the fourth caliph没有carry on as the fourth caliph来得简洁而且idiomatic,

谢谢bon！！

但是我觉得有时对于effectiveness方面的解释是因为知道答案，所以解释就会偏向正确答案，而在做题的第一反应中就不会想的那么细，往往是二选一时选错，像这题在做题时因为有一个时间状语in the late seventh century,我就认为was going to 过去时在时态上没有错，而should反而没有体现时态，所以将a排除，用通不通顺来解释总觉得有点牵强的，有没有其他的语法点可以更有力的说服呢??

was going to 的不通顺是这样的：

be going to 通常表示某人打算做某事，表示主观意愿；这里讨论的是某人是否要继位的问题，不是由他可以决定的。

所以用be going to 不妥。should 为什么不能体现事态呢？情态动词既能表示某种可能或意愿，也能表示事态，象could,might都是可以的。至于你说的对于effectiveness和correctness的反应问题，平时要加强这方面的训练：对于ets常考的逻辑点（主谓一致，平行结构，时态，代词等）要保持极高的敏感度，其他的语法点相比较这些应该为弱。就好像某个股票被国外某某机构列为“增持”或是“持有”一样，我们也要有这样的划分