A、Colombian. B、Peru. C、Bolivia. D、Vietnam. A细节题。根据文中两处，“Colombia is the source of most of the cocaine and heroin used in the U．S

Illegal drugs are produced on farms, in laboratories, and in backyards all across the world. Although millions of dollars worth of drugs are produced within our country, millions more also come from other countries on boats, planes and in drug smugglers’ suitcases.
    Colombia is the source of most of the cocaine and heroin used in the U.S. Drugs, and cocaine are the most widely trafficked drugs worldwide. Before Colombia became our leading supplier, farms in other South American countries grew acres and acres of coca plants, the raw material from which cocaine is made.
    The U.S. worked with the government of Peru and Bolivia to reduce the drug supply, spraying plant-killing chemicals on acres of drug plantations and paying farmers to grow other crops instead. Coca production was cut in half.
    The problem is, much of the drug production simply moved from those countries into neighbouring Colombia.
    The U.S. government is spending $1.3 billion on a detailed Long-Term Anti-Drug plan that helps the Colombian government get rid of its coca farms.
    However, Colombian farmers—most of whom are very poor and have few other opportunities for employment— say coca brings in the most money and they are unsure the Colombian government will keep its promises to help make other crops just as profitable.
Questions 19 to 21 are based on the passage you have just heard.
19. Where is the leading cocaine and heroin supplier?
20. How does the U.S. government solve the problem?
21. Why Colombian farmers chose to grow coca?