How Is It That Viruses Can Be Thought of as the "Bark

Multiple Choice

How is it that viruses can be thought of as the "bark" on the tree of life: an important component all across the tree, but not quite like the main branches?

A) Most plant viruses primarily cause a reaction on the outer surface of plant stems.
B) Viruses are smaller than any forms of cellular life.
C) Viruses are only of peripheral importance to life on Earth.
D) Replicating viruses often escape the host through the cell membrane.
E) They have evolved repeatedly from unrelated cellular organisms, not from one another.