Why Is the Sea Otter Considered a Keystone Species in Areas

Why is the sea otter considered a keystone species in areas of the Pacific Ocean?

A) Sea otters eat barnacles and mussels,and if sea otter populations were to crash,barnacles and mussel populations in tide pools would expand,causing a decrease in biodiversity in those areas.
B) Sea otters were introduced to control kelp and other invasive plants,and without them,the plants would grow too thick to support crab and lobster populations in the area.
C) If sea otter populations were to crash,kelp forests could disappear because of overgrazing by sea urchins that would otherwise be controlled by the sea otters.
D) If sea otter populations were to crash,sea urchins would be unable to survive because kelp forests would become too dense.