楼上的,请看og解释:Choice B is not assumed: the argument need only assume that many institutions depend heavily, but not necessarily exclusively, on donations from such individuals.
A 、B和D都可以算是assumption。但是,既然是选最优答案,只有A是正确的,因为B和D虽然也算是合理的assumption,但是他们却都是建立在A的基础上的,也就是说B和D不是最直接能够把条件和结论联系在一起的桥梁。当然,我个人认为,即便A作为assumption了,条件和结论之间还是有gap,A、B(或A、D)同时作为这题的assumption,才是最为严谨的逻辑。