Are species whose roles have a large effect on the types and abundance of other species in an ecosystem.

Keystone species are those species whole roles have a large effect on the types and abundance of other species in an ecosystem. For example; pollination of flowering plant species by butterflies, bees, hummingbirds, bats and other species. Top predator keystone species feed on and help regulate the populations of other species and American alligator. Another example;decline of certain shark populations lead to a explosion in populations of rays and skates, which sharks normally feed on. Now rays and skates are feasting on bay scallop fishing business. 

The answer is a if u read the definition it makes sense