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　　"I have great confidence that by the end of the decade we'll know in vast detail how cancer cells arise," says microbiologist Robert Weinberg, an expert on cancer. "But ," he cautions, "some people have the idea that once one understands the causes, the cure will rapidly follow. Consider Pasteur. He discovered the causes of many kinds of infections, but it was fifty or sixty years before cures were available."

　　This year, 50 percent of the 910,000 people who suffer from cancer will survive at least five years. In the year 2000, the National Cancer Institute estimates, that figure will be 75 percent. For some skin cancers, the five-year survival rate is as high as 90 percent. But other survival statistics are still discouraging - 13 percent for lung cancer, and 2 percent for cancer of the pancreas(胰腺)。

　　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."

　　53. The example of Pasteur in the text 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

　　[C] prove that cancer will be cured in fifty to sixty years

　　[D] warn that there is still a long way to go before cancer can be conquered

　　54. The author implies that by the year 2000, ____.

　　[A] there will be a drastic rise in the five-year survival rate of skin-cancer patients

　　[B] 90 percent of the skin-cancer patients today will still be living

　　[C] the survival statistics will be fairly even among patients with various cancers

　　[D] there won't be a drastic increase of survival rate of all cancer patients

　　55. Oncogenes are cancer-causing genes ____.

　　[A] that are always in operation in a healthy person

　　[B] which remain unharmful so long as they are not activated

　　[C] that can be driven out of normal cells

　　[D] which normal cells can't turn off

　　56. The word "dormant" in the third paragraph most probably means ____.

　　[A] dead

　　[B] ever-present

　　[C] inactive

　　[D] potential