Like wolves, beavers are ecosystem engineers. Why did the reintroduction of wolves to Yellowstone National Park benefit beavers and other species?

A. Beavers like to eat the trees that grew back along the river banks.
B. Beaver dams, built in the rivers, provide habitat for other species like otters, ducks, fish, reptiles, & amphibians.
C. Beavers feed on the carcasses of deer left by the wolves.
D. Beavers build their nests high up in the trees that quintupled in size in just a few years after the wolves were reintroduced.