A、30 percent. B、40 percent. C、50 percent. D、60 percent. A录音讲到美国过去的20多年间brain tumors增长了30 percent。

Millions of American baby boomers who received polio shocks in the 1950s and 1960s could now be at risk of getting cancer.
    That is the shocking finding of an American investigation into the vaccine that was giving to school children 40 years ago to fight the polio plague that was sweeping the nation. Experts now say that some of the injections were contaminated by a virus from monkeys, known as S.V 40, which may cause a rare form of cancer in humans. Batches of the vaccine were somehow contaminated by the virus between 1955 and 1963, and accidentally given to young children in 37 states. Moreover, some doctors believe that the virus may have already mutated and could now be transmitted through human contact, ultimately causing an epidemic, similar to AIDS.
    The terrifying finding was published in the prestigious cancer research journal which reported that a startling new rise in some types of cancer, such as brain tumors could be linked to the tainted vaccine. It points to SV 40 as a possible reason for a 30 percent increase in brain tumors in the United States over the past 20 years.
     “At first, it was just a finding of one more virus, just another thing we had to scream for. But then we started to develop tumors in hamsters,” said Doctor Morris Helaman. Doctors were shocked by this discovery which is being investigated in heavily populated area.
Question 21-25 are based on the passage you have just heard:
21. What happened 40 years ago in the United States?
22. What is SV 40?
23. What happened to young children in 37 American states 40 years ago?
24. How much has the incidents of brain tumor increased in the past twenty years?
25. What did the researchers discover in the investigation?