A fish swimming near the whirlpool's edge feels a gentle tug as the current drags it toward the spinning center. No problem. With a little swish, the fish can speed up, giving itself the oomph to swim away. After putting some distance between itself and the whirlpool, the fish no longer feels the current's inward pull.

But what if the fish drifted farther in?

Closer to the center of the whirlpool, the pull would grow stronger. To escape the whirlpool, the fish would have to swim faster than it had to at the edge. Even closer to the middle, if the fish couldn't go much faster, it would find itself swept all the way in, stuck in the swirl, pulled round and round and round.

A black hole works something like that whirlpool.

What is being compared or contrasted in this excerpt?

A comparison between a swimming fish and a black hole is being made.
A comparison between a whirlpool and a black hole is being made.
A contrast between a swimming fish and a black hole is being made.
A contrast between a whirlpool and a black hole is being made.