听力原文： Nowadays people build ponds mainly for fishing and swimming and for the pleasure of looking at water. No matter why the pond was built, one thing is certain: All kinds of plants and animals will soon move in. A pond is a food factory and a restaurant, open 24 hours a day. Millions of plants and animals make the food. Some of the animals in a pond eat nothing but plants. Some eat nothing but other animals. Some eat both plants and animals, depending on what's handy. As a pond gets older, it gets richer. The mud on the bottom turns black. Millions of dead plants and animals make it black. Pond life depends on death, for dead plants and animals help new ones to grow. (30)