题目

Dolphins (海豚) are not fish, but warm-blooded animals. They live in groups, and speak to each other in their own language. In this way they are like other animals, such as bees and birds. But dolphins are very different from almost all land animals. Their brain is nearly the same size as our own, and they live a long time—at least twenty or thirty years. Like some animals, dolphins use sound to help them find their way around. They also make these sounds to talk to each other and to help them find food. We now know they do not use their ears to receive these sounds, but the lower part of 中，，，则（     ），（     ）