In the Context of Earth's Climate History,what Do We Mean

In the context of Earth's climate history,what do we mean by snowball Earth?

A) This term is used to describe all the ice ages that have occurred in the past few million years.
B) It refers to a time when polar regions had much more snowfall than normal.
C) It is what we expect to see happen on Earth in about a billion years.
D) It refers to a very deep ice age that occurred hundreds of millions of years ago.