题目内容：

    Dolphins are not fish. They are mammals. They live in groups and speak to each other in their own language. But they aren't the same as almost all land animals. Their brains are as big as men's and they live a long life--- at least 20 or 30 years.

Like some animals, dolphins use some sounds to help them find their ways around. They also use sounds to talk to each other and to help them find food. They don't use their ears to receive sounds, but use the jaws(下颚).

It's strange that dolphins are friendly to man, and for thousands of years, there are many stories about dolphins and man. In the 19th century, in a dangerous part of the sea near New Zealand, there was a dolphin. The seamen liked him very much and gave him a very good name, Jack. From 1871 to 1903, Jack met every ship in the area and showed it the way. In 1903, a passenger on a ship with the name of " The Penguin" shot and hurt Jack. Luckily the dolphin didn't die, and after a long time he was well again. He guided the ships through the area for nine more years, but he refused to help the ship " The Penguin".

Today, some people still kill dolphins, but more people in the world protect them.

根据短文内容，回答下列问题。

1.Are dolphins fish or mammals?

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2.What do dolphins use to receive sounds?

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3.Where did Jack live?

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4.Why did Jack refuse to help the ship "The Penguin"?

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5.How many years did Jack guide the ships through the area in total(总共)?

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