B. A comparison between a whirlpool and a black hole is being made.
Explanation:
In this text, the author explains how a whirlpool works. He uses the example of a fish in order to explain this mechanism to the reader. The author describes the way the fish would have to swim in order to resist the force of the whirlpool. The author then tells us that the mechanism the whirlpool uses is similar to that of a black hole. Therefore, the items being compared are a whirlpool and a black hole.
