A) They are single-stranded molecules that can fold into very specific shapes.
B) They are single-stranded molecules that cannot fold into very specific shapes.
C) They are double-stranded molecules.
D) They are double-stranded molecules that can fold into very specific shapes.

A) The rust layers in them serve as evidence for the presence of rocks on early Earth.
B) The rust layers in them serve as evidence for the presence of iron on early Earth.
C) The rust layers in them serve as evidence for the fall of atmospheric oxygen on early Earth.
D) The rust layers in them serve as evidence for the rise of atmospheric oxygen on early Earth.

A) about 4.6 million years old
B) about 1 billion years old
C) about 3.6 billion years old
D) about 4.6 billion years old