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 supposedly because its
(B) by whom they were named supposedly and their
(C) naming them supposedly since their
) who so named them supposedly because their
(E) who so named it since supposedly their
My question: is it correct to have comma placed before "by whom".
What grammar is related to this structure "JC, by whom?