呼吸的全过程包括下面四个环节，其正确顺序是（　　）

①肺内气体交换  ②组织内气体交换  ③肺通气  ④气体在血液中的运输．

（2012·宁波）如图，倾斜轨道上的滚动体由A处自由释放，向B处滚动(不计摩擦及空气阻力)，所受重力的方向____ 。有三位同学分别对此过程作了以下判断，你认为正确的是____ (选填序号)。   ①滚动体在A处受力平衡    ②滚动体在B处的势能小于A处的势能    ③滚动体在A处的动能大于B处的动能

用句中加下划线的词语造句

岛上有一片片茂密的树林，树林里栖息着各种海鸟。

    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.