Which is most likely a keystone species?

Question

Responses

wolves in Yellowstone National Park, which eat elk, who graze on willows, on which beavers depend to build their dams
wolves in Yellowstone National Park, which eat elk, who graze on willows, on which beavers depend to build their dams - correct

deer, which eat a variety of plants, including shrubs and young trees, and are consumed by large predators like mountain lions, wolves, and bears
deer, which eat a variety of plants, including shrubs and young trees, and are consumed by large predators like mountain lions, wolves, and bears - no response given

cane toads in Australia, which were introduced to eat cane beetles, but eat anything and survive in various habitats
cane toads in Australia, which were introduced to eat cane beetles, but eat anything and survive in various habitats - no response given

feral cats, which hunt a wide variety of animals, including birds, lizards, rodents, and insects

Answer 1

feral cats, which hunt a wide variety of animals, including birds, lizards, rodents, and insects - no response given

Answer 2

this answer correct

Answer 3

No, that answer is not correct. The correct response is: wolves in Yellowstone National Park, which eat elk, who graze on willows, on which beavers depend to build their dams. Wolves in this ecosystem are considered a keystone species because they have a significant impact on the ecosystem by controlling the elk population, which in turn affects the behavior of other species such as beavers and plants like willows.