While the most abundant and dominant species within a particular ecosystem is often crucial in perpetuating the ecosystem, a "keystone" species, defined as one whose effects are much larger than would be predicted from its abundance, can also play a vital role. However, identifying a keystone species by removing it and observing changes in the ecosystem is problematic due to complex species interactions.

For example, Pisaster ochraceus is often a keystone predator because it consumes and suppresses mussel populations, which, in the absence of this starfish, can become a dominant species. However, such predation on a dominant or potentially dominant species occurs in systems with and without species playing keystone roles.

Moreover, while P. ochraceus occupies an unambiguous keystone role on wave-exposed rocky headlands, in more wave-sheltered habitats, the impact of P. ochraceus predation is weak or nonexistent, and sand burial is sometimes responsible for eliminating mussels. Keystone status appears to depend on context, whether it be geography, community diversity (such as a reduction in species diversity thrusting more remaining species into keystone roles), or the length of species interaction (since newly arrived species may dramatically affect ecosystems).

Which of the following factors affects whether P. ochraceus has a keystone role in a habitat?

A. The degree to which the habitat is sheltered from waves
B. The degree to which other animals within a habitat prey on mussels
C. The fact that mussel populations are often not dominant within some habitats occupied by P. ochraceus
D. The size of the P. ochraceus population within the habitat
E. The fact that there is great species diversity within some habitats occupied by P. ochraceus