People, i know that option D looks parallel and the idiom is correct, but my question what excatly is/are the mistake(s) in option A
Option A: Scientists who studied the X known as Y concluded that <subordinate clause-passive voice> but that <subordinate clause-active voice>
Both the parts after 'that' can be considered parallel, as both are clauses and form parts of what the scientists concluded.
Is it inferior to have active and passive structures such as these?
Also can someone comment on the position of 'over millions of years'; i think this phrase is mis-placed in option A.