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

OA=D

I answered this correctly. I'm just curious to see what other logic can be used to arrive at the same answer.