Answer:
A. ability to find a mate
Step-by-step explanation:
A river already having several dams built on it indicates existence of beaver population in that region. With a large population, more options are present to the beaver for choosing their mate.
Beavers are large rodents that thrive near the water bodies. They build dams on water to decrease the speed of flowing water so that their shelter is safe from water destruction.
Alternatively, this region provides a safe shelter. The safety also helps a beaver in attracting a mate.