This lake is called Beaver Lake, but until 10 years ago there hadn't been any beavers living in it for decades. What is the impact of these animals on the lake ecosystem? a) Beavers are invasive species whose dams block the lake from naturally draining. b) Beavers are invasive species that cut down a large number of trees, contributing to the infilling of the lake. c) Beavers were hunted extensively in the park and were reintroduced 10 years ago by park ecologists. d) Beavers improve the ecosystem by digging up sediment and clearing invasive species.