HealthofficialsinCanadaareverybusythesedays.Theyareplac...

问题详情：

Health officials in Canada are very busy these days. They are placing chickens at fixed points all along their border with the United States of America. They are using these chickens to see if the deadly West Nile virus exists in their country. The virus spreads to birds, so they think that the chickens have a good chance of catching the virus.

The Canadians are worried because the West Nile virus is passed to humans by mosquitoes. It killed seven people in New York last year. Countries around the world are realizing something important—it may just be possible to stop certain kinds of people from entering their land; but it is very difficult to try and stop viruses that travel from one end of the earth to another. When they travel to new places, they adjust very easily to those climates and sometimes start destroying the local plant and animal life. These biological polluters are called smart polluters.

These smart polluters can be carried across borders of countries unknowingly. Just as we humans are travelling across the globe more often than earlier, these biological polluters have also started journeying much more. They travel in many things carried from one country to another.

 There are some good examples of how these polluters work. The water hyacinth(水葫芦) of South America is filling lakes in China and Africa. Tree snakes from Papua New Guinea are busy eating up bird in the country of Guam, which is quite far away. Tiger mosquitoes from Asia are busy biting Americans.

Suddenly, a lot of things become clearer. Like why the customs(海关) officials in many foreign countries prevent you from bringing in a small plant, they tell you that it’s in the rules. They have these rules because they know that these kinds of plants that are special to specific places have the power to spread new diseases among native plants and animals. They are called biological polluters and they always create problems in places where they do not belong . They could be special kinds of plants of animals.

28. According to the text, the West Nile virus______.

   A. has existed in Canada          B. is dangerous to humans

   C. spreads from birds to humans    D. is unable to survive in cold climates

29. What’s the characteristic of smart polluters?

   A. They are quite tiny

   B. They are as smart as humans.

   C. They travel with people around the world

   D. They get used to the new environment easily

30. The purpose of the examples in Paragraph 4 is to show that_____.

   A. a certain kind of mosquito can cause death

   B. biological polluters have traveled around the world

   C. it’s impossible to bring biological polluters to an end

   D. plants as well as animals can damage the environment

31. Why are plants not allowed to be taken to a foreign country?

   A. They can’t adjust to the new climates

   B. They can’t live together with the native plants

   C. They will destroy the balance of the local nature

   D. They may spread a certain virus to the native plants.

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