E. Polar bears have to hunt a lot of time because ... 1) they swim a lot. 2) they run a lot. 3) they need to be fat. 4) they must be at the top of the food chain.

The reason polar bears have to hunt a lot is primarily because they need to maintain a high level of body fat to survive in their cold environment and to sustain their energy levels for hunting. While swimming and running are part of their hunting behavior, and they are at the top of the food chain, the necessity of being fat is a direct consequence of their lifestyle and environment that drives their hunting. Therefore, option 3 is the most direct cause for extensive hunting.