Most people are afraid of sharks，but they usually do not know very much about them．For example，there are 350 types of sharks，and all of them are meat eaters．Some sharks are very big．

The whale shark is 50 to 60 feet long．But some sharks are very small．The dwarf（侏儒）shark is only 6 inches（英寸）long．Sharks are 100 million years old．In fact，they lived at the same time as dinosaurs．Today，sharks live in every ocean on the earth，but most sharks live in warm water．They keep the oceans clean because they eat sick fish and animals．Most sharks have four to six rows of teeth．When a shark's tooth falls out，another tooth moves in from behind．Sharks do not have ears．

However，they "hear" sounds and movements in the water． Any sound or movement makes the water vibrate． This helps sharks find food． Sharks use their large eyes to find food，too． Most sharks see best in low light． They often hunt for food at dawn（黄昏），in the evening，or in the middle of the night． Scientists want to learn more about sharks for several reasons． For example，cancer（癌症）is common in many animals，including people． However，it is rare（罕见的）in sharks． Scientists want to find out why sharks almost never get cancer． Maybe this information can help people prevent cancer，too．

（1）According to the text，most sharks live 　　．

A．	in very deep water
B．	in warm water
C．	near the surface of the ocean
D．	in cold water

（2）Sharks keep the oceans clean because they 　　．

A．	do well in cleaning water
B．	eat sick fish and animals
C．	can get cancer like human
D．	can produce a kind of chemical

（3）What does the underlined（划线的）word "vibrate" mean in Chinese？ 　　

A．	振动
B．	停止
C．	变清澈
D．	变浑浊

（4）According to the text， we know 　　．

A．	the whale shark is 50 to 60 feet long
B．	the dwarf shark is only 6 feet long
C．	sharks and dinosaurs lived in different times
D．	sharks only use large eyes to find food

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