OG12 SC01-C

The Glass House Mountains in Queensland, Australia, were sighted in 1770 by the English navigator Captain James Cook, by whom they were named supposedly because its sheer wet rocks glistened like glass.
A. by whom they were named supposely because its
B. by whom they were named supposely and their
C. naming them supposely since their
D. who so named them supposely because their
E. who so named it since supposely their
这个我也不是很懂哎~~   答案上说这里since是介词，介词后面不是不能跟完整的句子么？但是since有这样的用法，Since you are unable to answer, perhaps we should ask someone else. 而且字典上解释since也可以做连词，那在这句里面，since是连词还是介词？怎么判断出来在C选项里since就是介词呢？ 如果不考虑表示原因的强度，用since替换正确答案D中的because在语法上正确么？我排除C答案主要是根据naming 和 觉得supposely 修饰since很别扭，since在这里的用法还是不清楚啊~
求牛人精解呀~！！！谢谢谢谢~~

刚查了朗文，since作介词的用法，只有“自从”这一解释，难道GMAC把since在这里理解为“自从”啦？


另外，我不明白D选项so和because同时出现难道不重复吗？

so 在这里并不是表示原因的“因为”的意思，而是“如此，这样”的意思吧，who so named them翻译为“这样命名他们的那个人” ，与because  不冲突的。
另外c选项解释里把since当成介词这一点我也不是很懂。