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  3. Despite the fact that today virus are known to cause cancer in animals and in plants, there exists a great reluctance to accept

Despite the fact that today virus are known to cause cancer in animals and in plants, there exists a great reluctance to accept

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问题     Despite the fact that today virus are known to cause cancer in animals and in plants, there exists a great reluctance to accept viruses as being of importance in human cancer. Basic biological phenomena generally do not differ strikingly as one goes from one species to another. It should be recognized that cancer is a biological problem and not a problem that is unique for man. Cancer originates when a normal cell suddenly becomes a cancer which multiplies widely and without apparent restraint. Cancer may originate in many different kinds of cell, but the cancer cell usually continues to carry certain traits of the cell of origin. The transformation of a normal cell into a cancer cell may have more than one kind of cause, but there is good reason to consider the relationships that exist between viruses and cancer.
    Since there is no evidence that human cancer, as generally experienced, is infectious, many persons believe that because viruses are infectious agents they cannot possibly be of importance in human cancer. However, viruses can mutate and examples are known in which a virus that never kills its host can mutate to form a new strain of virus that always kills its host. It does not seem unreasonable to assume that an innocuous latent virus might mutate to form a strain that causes cancer. Certainly the experimental evidence now available is consistent with the idea that viruses as we know them today could be the causative agents of most, if not all cancer, including cancer in man.
It is possible that viruses cause human cancer because______.
选项 A、Human cancer is infectious.
B、Human cancer has more than one kind of cause.
C、Man is host to many viruses.
D、Viruses can mutate.
答案D
解析 根据文章最后一段倒数第二句“It does not seem unreasonable to assume that aninnocuous latent virus might mutate to form a strain that causes cancer．”可知，我们有理由假设一个不具危害性的潜在病毒可能会变异成另一种能够引发癌症的新型病毒。据此判断，本题选D。
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