What is one major reason that the earliest organisms on Earth were confined to the oceans?

A) The oxygen in the atmosphere was toxic.
B) There was not enough oxygen in the atmosphere to support life.
C) Not enough ultraviolet light could penetrate the atmosphere to support life.
D) The oceans created a shield against the damaging effects of ultraviolet radiation.
E) The salt within the oceans provided the raw materials for making proteins.