For some reason, I don't understand what it's saying. Help?

Tropical rain forests have the greatest biodiversity of any type of land ecosystem. How does biodiversity contribute to the sustainability of an ecosystem?

A: The presence of more species with different adaptations makes it more likely that some organisms will survive an ecological disaster.

B: Greater genetic variation within species makes it more likely that some individuals will survive a disease outbreak.

C: The presence of a variety of herbivore species that can feed on a larger number of different producer species helps ensure abundant prey for predators in the ecosystem.

D: All of the above