Sentence Correction

A little under a million years ago, the briny waters of the Baltic Sea began flooding into the cold North Atlantic: geologists are still debating whether the flood was gradual or created a cataclysm.
(A) whether the flood was gradual or created a cataclysm
(B) if the flood was gradual or created a cataclysm
(C) about whether the flood was gradual or cataclysmic
(D) whether the flood was gradual or cataclysmic
(E) whether the flood was gradual or it created a cataclysm


The OA is D. Can someone explain to me what is wrong with A.

here's what I think :

"geologists are still debating whether the flood was igraduali or icreated a cataclysm.i "

It is unclear if the flood was caused by a cataclysm or caused a cataclysm.

Answer choice D clearly states the correct meaning of the sentence. This is a case of parallel construction. the flood was - gradual or cataclysmic.