OG 12-1 since和because的用法

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 we 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

Agreement; Rhetorical construction

To avoid a wordy and confusing series of passive classes, the relative clause explaining what James Cook did should be an active-voice construction (who so named rather than by whom they were named).  The possessive pronoun referring to the Glass House Mountains should be plural (their rather than its), to agree with the plural antecedent.

A. By who they were supposedly named is a passive construction that is unnecessarily indirect and wordy, especially immediately following another passive construction; the singular its does not agree with the plural antecedent the Glass House Mountains.

B. This version of the sentence loses the causal connection, failing to explain why James Cook gave the mountains their particular name.

C. As the object of a preposition and not the subject of the clause, James Cook does not work as the noun that the verbal phrase beginning with naming can describe; the preposition since loses the important causal logic of the sentence.

D. Correct. This concise sentence uses active-voice construction in the relative clause and maintains agreement between the pronoun their and its antecedent.

E. The pronoun it does not agree with the plural Mountains and the following pronoun their.

选项C中间，说"the preposition since loses the important causal logic of the sentence."

这个怎么理解，请NN指点。

哪道题、

请牛牛说一下，谢谢