Sharks are the most frightening animals in the sea. Sometimes we can see sharks swimming in the sea around Hong Kong. People put nets around some beaches to protect swimmers. When there is a shark's flag, people must leave the beach. It will be too late if a shark can swim 60 kilometers an hour to you. 
There are 30 kinds of sharks and 25 kinds of them are dangerous. The whale sharks are very big. They can grow to 18 meters long, but they are not dangerous to people. The jaws and the tiger sharks are the most dangerous kinds of sharks. 
All sharks must keep swimming, or they will go down. All sharks can feel everything that happens around them. We must keep calm and don't make any noise when a shark is swimming near you. If a shark loses a tooth, a new one will grow out very quickly. So sharks always have many sharp teeth.

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1.Why do people put nets around some beaches?

A To give sharks food there.

B To ask people to swim.

C To keep swimmers safe.

D To keep sharks swimming.

解析：选C。细节理解题。根据文章第一段第三句People put nets around some beaches to protect swimmers.可知，人们在海滩上铺网是为了保护游泳者，故选C。

2._______ are the most dangerous kinds of sharks.

A The jaws and the tiger sharks

B The whale sharks and the tiger sharks

C The jaws and the whale sharks

D The whale sharks and the blue sharks

解析：选A。细节理解题。根据文章第二段最后一句The jaws and the tiger sharks are the most dangerous kinds of sharks.可知，大白鲨和虎鲨是最危险的鲨鱼，故选A。

3.The underlined words “keep calm” mean _______.

A 一直吵闹

B 保持镇静

C 不停尖叫

D 任性胡闹

解析：选B。词义猜测题。根据所在划线词下文don't make any noise when a shark is swimming near you可知，鲨鱼在你附近游泳时不要发出任何声音，由此可知我们应该保持“镇定”，故选B。

4.Which is NOT true?

A There are 30 kinds of sharks in the sea.

B Sharks' teeth can grow out very quickly.

C It is dangerous if a shark swims fast to you.

D All sharks can stop swimming in the sea.

解析：选D。细节理解题。根据文章第三段第一句All sharks must keep swimming, or they will go down.可知所有的鲨鱼必须一直游泳，否则会掉下海底，故选D。