Much of the World's Supply of Iron Comes from Banded

Much of the world's supply of iron comes from banded iron deposits that formed between 1.8 and 2.6 billion years ago. In addition to being a source of iron, why are these deposits important in geology?

A) represent the point in time when free oxygen was becoming more abundant
B) provide proof of the first known ice ages
C) mark the point in history when marine mammals were first beginning to evolve
D) provide evidence of the earliest known sedimentary rocks
E) all of the answers listed here