cer 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 pan- creas (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 disoovered that oncogenes, which are cancer-causing genes (基因) , are inactive in normal cells. Anything from cosmic rays to radi-
ation 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 nor- mal 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 deter- mine whether there are a limited number of genes in cells which are always responsible for at leasl 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
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
64. The author implies that bv 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
65 . 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
66 . The word "dormant" in the third paragraph most probably means__.
A. dead B. ever-present C. inactive D. potential
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Passage 5
Discoveries in science and technology are thought by "untaught
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