What Was the Root Cause of the Decline of Kelp

What was the root cause of the decline of kelp beds on the west coast of the U.S.?

A) Overfishing of the perch and herring in the area.
B) A decline in the sea otter population that would control the sea urchin population.
C) An increase in the orca population preying upon the sea otters.
D) An explosion of the sea urchin population.
E) A decline in the seal and sea otter population.