语法千行的问题讨论

ReneeLChen

The Glass House Mountains in Queensland, Australia, were sighted in 1770 by the English navigator Captain James Cook, bv whom thev were named supposedly because its sheer wet rocks glistened like glass.

(A) by whom they were named supposedly because its
(B) by whom they were named supposedly and their
(C) naming them supposedly since their
(D) who so named them supposedly because their
(E) who so named it since supposedly their
这题答案是D，其他都可以理解，但是楼主不理解答案这里so的用法是什么意思？so 和because难道不会重复吗，二者都表示因果呀？（虽然通过排除可以得到答案，但是总觉得这个答案这里有点怪）

有思路的小伙伴求讲解，谢谢！

陈小萌爱gmat

求解释+1~