[SC悬赏令] 急！向大佬请教这里so和because为什么不重复？

The Glass House Mountains in Queensland,Australia, weresightedin 1770 by the English navigator Captain James Cook, bv whom thev werenamed supposedly because its sheer wet rocksglistened like glass.  

(A)        by whom they werenamed supposedly because its

(B)       by whom they werenamed supposedly and their

(C)       naming them supposedlysince their

(D)       who so named themsupposedly because their(D)