A、Its skeleton. B、Its shape. C、Its senses. D、Its body temperature. B这段话里重点讨论了水的不可压缩性对鱼的影响，正是水的不可压缩性决定了鱼的外形进化，可见，说话者讨论最多的应该是鱼的外形。

To us, the environment in which fish dwell often seems cold, dark, and mysterious. But there are advantages to living in water, and they have 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:One, a fish can get along with a light weigh and simple bone structure, and two, limitations to a fish’s size are practically removed.
    Yet there is one basic difficulty to living in water—the fact that it’s incompressible. For a fish to move through water, it must actually shove it aside. Most can do this by wiggling back and forth in snakelike motion. The fish pushes water aside by the forward motion of its head, and with the 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.
10. What is the talk mainly about?
11. What does the speaker mention as a problem that water presents to fish?
12. The speaker compares a fish’s movement with that of what creature?
13. What aspect of a fish does the speaker discuss in the most detail?