资料题：The word"rock" in the fourth paragraph refers to（　）。

Fish are animals that live in water.They live in almost any place that there is water.Some arefound in lakes.Other fish live in the sea.Moet fish never leave the water.There are about21 ,000 kinds of fish.One kind might not look like another.Some fish are very small.The smallest one is no bigger than a fly.Others are very big.The biggest fish grows up to 60 feet.Fish come in many colors.They can be blue, black, or white.Some are red.Others are green.Many have stripes or spots.These are very pretty.Many fish have a dark color over a light color.Tuna are colored like this.They have dark blue over white.Fish come in many shapes, too.Some fish are very long.They look like snakes.Others look like the plants they live near.Still others look like rocks.They sit on the sea floor.Then they wait.Soon smaller fish swim near.The smaller fish think they are swimming by rocks.But these"rocks" eat them.One kind of fish looks as if it has a little horse's head.It is called a sea horse.It doesn't swim very well.A sea horse is mostly pushed along by the moving water.What does it do when it wants to stay in one place? it takes hold of plants with its little tail.