听力原文： (20) To most of us, sharks are the most dangerous fish in the sea and they attack humans. However, according to Doctor Clark, who has studied the behavior. of sharks for twelve years, humans are not normally on the sharks' menu. What do sharks feed on, mainly fish and other sea animals? (18) Doctor Clark also found that sharks don't eat as much food as people think. For instance, a nine-year-old shark only needs two pounds of food a day to keep healthy. But she says, sharks sometimes starve and at other times they fill themselves with what they have killed. Around the world, there are only about one hundred shark attacks on humans each year, ten of which proved fatal. But consider this, in the U. S. alone, about three million people are bitten by dogs each year. Of these, thirty people die. If sharks bite you, says Doctor Clark, the reason is usually because of mistaking you for its natural food. For example, say you went underwater-fishing and saw a shark, you could be in trouble. The shark might go for the injured fish you had attacked and take a bite of you at the same time. (19) If you go into a shark's territory and threaten it, it might try to bite you. That's because sharks are territorial and tend to guard their territory. Like dogs, they protect the area they think is their own. What does the passage say about the eating behavior. of sharks?