[答案]　A

　　Bananas are one of the world’ s most important food crops. They are also one of the most valuable exports. Bananas do not grow from seeds. Instead, they grow from existing plants. Bananas are threatened by disease because all the plants on a farm are copies of each other. They all share the same genetic weaknesses. For example, the Cavendish banana is most popular in North American and European markets. However，some kinds of fungus organisms easily infect the Cavendish. Black Sigatoka disease affects the leaves of Cavendish banana plants. The disease is controlled on large farms by putting chemicals on the plant’ s leaves. Farmers put anti-fungal chemicals on their crops up to once a week.

　　Another fungal disease is more serious. Panama disease attacks the roots of the banana plant. There is no chemical treatment for this disease. Infected plants must be destroyed. Panama disease has affected crops in Southeast Asia, Australia and South Africa. There is concern that it may spread to bananas grown in the Americas. This could threaten an important export product for Central and South America.

　　The International Network for the Improvement of Banana and Plantain supports research on bananas. The group has headquarters in France and other offices in the major banana-growing areas of the world. The group says that more research must be done to develop improved kinds of bananas. The group says that fungal diseases mainly affect only one kind of banana. In fact, there are five hundred different kinds of bananas. Food and Agriculture Organization of the United Nations has said that the Cavendish banana represents only 10% of world production.

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