A、The Black Plague. B、Enemy of Humanity. C、Common Animal pests. D、Causes of World Hunger. B主旨题，文章开头便指出Rats are the enemy of huma

Rats are the enemy of human beings. They eat or spoil crops of grain and rice before they can be harvested, or while they are in storage. In India, where millions of people go hungry, there are ten times as many rats as people. Rats devour half of the available food. Rats will also attack birds and animals, from frogs and chicks to geese and young calves. They have even destroyed dams and buildings by making holes through or under them, and have started fires by chewing on electrical wiring.
    The most terrible destruction caused by rats, however, has come from the diseases they carry. In the fourteenth century, rats caused the death of one-third of the world’s population by transmitting the dreadful Black Plague that mined Europe.
    Ironically, it is in fighting diseases that rats have been most useful to humanity. Thousands of specially bred rats are used in research laboratories every year to test medicines which can possibly be used to prolong and improve human life. Some laboratory rats are even used to test new methods of eliminating their cousins, the wild rats.