What Does the Observation That a Whale's Front Flippers Have

What does the observation that a whale's front flippers have the same bone structure as all mammalian forelimbs suggest?

A) All mammals descended from a common ancestor.
B) Whales once walked on land.
C) Whales show remarkable diversity.
D) Land mammals originally came from the sea.
E) There must have been land and aquatic ancestors that coevolved.