Orcas, or killer whales, are natural predators of sea otters. it has been observed that in regions where the sea otters are not present, sea urchins grow rapidly and destroy much of the natural kelp vegetation.

which of these statements is correct in this context?
a) kelp is the keystone species because their survival is key to the survival of the entire ecosystem.
b) sea otters cannot be considered the keystone species because they are not the top predators in the ecosystem.
c) sea urchins are the keystone species because they multiply rapidly in the absence of their natural predators.
d) sea otters are a keystone species as they prevent a herbivorous species from completely destroying a plant species.