With as many as 120 varieties in existence, discovering how cancer works is not easy. The researchers made great progress in the early 1970s, when they discovered that oncogenes, which are cancer-causing genes (基因), are inactive in normal cells. Anything from cosmic rays to radiation to diet may activate a dormant oncogene, but how remains unknown. If several oncogenes are driven into action, the cell, unable to turn them off, becomes cancerous.

　　The exact mechanisms involved are still mysterious, but the likelihood that many cancers are initiated at the level of genes suggests that we will never prevent all cancers. "Changes are a normal part of the evolutionary process," says oncologist William Haywar. Environmental factors can never be totally eliminated; as Hayward points out, "We can’t prepare a medicine against cosmic rays."

　　The prospects for cure, though still distant, are brighter. "First, we need to understand how the normal cell controls itself. Second, we have to determine whether there are a limited number of genes in cells which are always responsible for at least part of the trouble. If we can understand how cancer works, we can counteract its action. "

　　63. The example of Pasteur in the passage is used to__.

　　[A] predict that the secret of cancer will be disclosed in a decade

　　[B] indicate that the prospects for curing cancer are bright

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