A、A comparison of fish to warm-blooded animals. B、The difference between saltwater and freshwater environments. C、The importance

To us, the environment in which fish dwell often seems cold, dark and mysterious. But there are advantages to living in water.(22)And they’ve played an important role in making fish what they are. One is that water isn’t subject to sudden temperature changes. Therefore it makes an excellent habitat for a cold blooded animal. Another advantage is the water’s ability to easily support body weight. Protoplasm has approximately the same density as water. So a fish in water is almost weightless. This weightlessness in turn means two things:
    1)A fish can get along with a light weight and a simple bone structure. And
    2)Limitations to a fish’s size are practically removed.(20)Yet there is one basic difficulty to living in water—the fact that it is incompressible. For a fish to move through water, it must actually shove it aside.(21)Most can do this by wiggling back and forth in snake-like motion. The fish pushes water aside by the forward motion of its head and with a curve of its body and its flexible tail. Next the water flows back along the fish’s narrowing sides, closing in at the tail and helping the fish propel itself forward. The fact that water is incompressible has literally shaped the development of fish. A flat and angular shape can be moved through water only with difficulty. And for this reason, fish have a basic shape that is beautifully adapted to deal with this peculiarity.
20. What does the speaker mention as a problem that water presents to fish?
21. The speaker compares a fish’s movement with that of what creature?
22. What is the talk mainly about?