It is hard to get people to think of
sharks as anything but a deadly enemy. They are thought to {{U}} {{U}}
1 {{/U}} {{/U}}people frequently. But these fish perform a {{U}}
{{U}} 2 {{/U}} {{/U}}service for earth's waters and for human
beings. Yet business and sport fishing are threatening their {{U}} {{U}}
3 {{/U}} {{/U}}. Some sharks are at risk of disappearing from {{U}}
{{U}} 4 {{/U}} {{/U}}. Warm weather may influence
both fish and shark activity. Many fish swim near coastal areas {{U}}
{{U}} 5 {{/U}} {{/U}}their warm waters. Experts say sharks may
follow the fish into the same areas, {{U}} {{U}} 6 {{/U}}
{{/U}}people also swim. In fact, most sharks do not purposely charge at or bite
humans. They are thought to mistake a person {{U}} {{U}} 7
{{/U}} {{/U}}a sea animal, such as a seal or sea lion. That is why people
should not swim in the ocean when the sun goes down or comes up. Those are the
{{U}} {{U}} 8 {{/U}} {{/U}}when sharks are looking for food.
Experts also say that bright colors and shiny jewelry may cause sharks to
attack. A shark has an extremely good sense of smell. It can
find small amounts of substances in water, such as blood, body liquids and
{{U}} {{U}} 9 {{/U}} {{/U}}produced by animals. These powerful
{{U}} {{U}} 10 {{/U}} {{/U}}help sharks find their food. Sharks
eat fish, any {{U}} {{U}} 11 {{/U}} {{/U}}sharks, and plants
that live in the ocean. Medical researchers want to learn more
about the shark's body defense and immune {{U}} {{U}} 12
{{/U}} {{/U}}against disease. Researchers know that sharks {{U}} {{U}}
13 {{/U}} {{/U}}quickly from injuries. They study the shark in hopes of
finding a way to fight human disease. Sharks are important for
the world's {{U}} {{U}} 14 {{/U}} {{/U}}. They eat injured and
diseased fish. Their hunting activities mean that the numbers of other fish in
ocean waters do not become too {{U}} {{U}} 15 {{/U}} {{/U}}.
This protects the plants and other forms of life that exist in the
oceans.
【答案解析】[解析] 第三句说到鲨鱼有益于“waters and human beings”，第五句又说到鲨鱼濒临灭绝的危险。根据上下文的意思，threatening(威胁)到它们的应该是existence(生存)。其他3个选项source，friends，fish都不合适，与上下文意思没有联系，故选B。