The reason why the winged bean can thrive on poor soil is because their roots house bacteria that can turn atmospheric nitrogen into fertilizer.
(A) is because their roots house bacteria that can turn
(B) is because its roots house bacteria which can turn
(C) is that their roots house bacteria turning
(D) is that its roots house bacteria that can turn
(E) being that its roots house bacteria and can turn

[spoiler]OA D.
Was mixed between B/D. What's wrong with B ?
The reason is because - is this structure shows redundancy ?[/spoiler]