根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。
Many people say dolphins (海豚) are smart. They seem to be able to think understand and learn things quickly. But are they as smart as humans? .
Like humans, every dolphin has its own "name". The name is a special whistle (哨声). .
Dolphins are like people in other ways, too. They "talk" to each other about a lot of things — such as their ages, their feelings, and finding food. No one understands dolphin language yet, but some scientists are trying to learn.
They live in groups, and they often join others to play games and have fun — just like people. Scientists believe playing together is something only smart animals do.
Dolphins and humans are similar in another way: both of the two make plans for getting things they want. In the seas of southern Brazil, for example, dolphins use a clever plan to get food. When there are fish near a boat, dolphins signal (发信号) to the fishermen to put their nets (网) in the water. Then the dolphins can get to eat some of the fish that swim away from the net.
A. Dolphins are also social animals.
B. Using this method, the men can catch a lot of fish.
C. Scientists say dolphins and humans are very similar in some ways.
D. Each dolphin chooses a certain whistle for itself on its first birthday.
E. However, understanding dolphin's conversation is not easy for humans.