Do you know that fish come out of eggs when they were born? After the baby fish comes out of the egg，it eats the food in the egg. When it is big enough，it leaves the egg. It swims in the water. You can see the eyes and the nose. Its ears are hidden (藏) in the head.

    Some fish eat other fish. The fish that eat plants have smaller mouths and teeth. The fish that eat other fish need larger mouths and stronger teeth.

Fish are usually dark on the top and light on the bottom.

下列表述不正确的一项是（    ）

“活水围网”养鱼技术具有养殖密度高、经济效益好的特点．研究表明：“活水围网”养鱼时，某种鱼在一定的条件下，每尾鱼的平均生长速度v（单位：千克/年）是养殖密度x （单位：尾/立方米）的函数．当x不超过4尾/立方米时，v的值为2千克/年；当4＜x≤20时，v是x的一次函数，当x达到20尾/立方米时，因缺氧等原因，v的值为0千克/年．

（1）当0＜x≤20时，求v关于x的函数表达式；

（2）当养殖密度x为多大时，鱼的年生长量（单位：千克/立方米）可以达到最大？并求出最大值．