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  3. Although the fact that today viruses are known to 【S1】______ cause cancer in animals and in certain plants, tha

Although the fact that today viruses are known to 【S1】______ cause cancer in animals and in certain plants, tha

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问题     Although the fact that today viruses are known to                  【S1】______
cause cancer in animals and in certain plants, that exists             【S2】______
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 through                 【S3】______
another. It should be recognize that caner is a biological             【S4】______
problem and not a problem that is unique for man.
Cancer originates when a normal cell suddenly becomes
a cancer cell which multiplys widely and without                       【S5】______
apparent restraint. Cancer may originate in many different
kind of cell, but the cancer cell usually continues                    【S6】______
to carry certain traits of the cell of origin. The
transformation of a normal cell into a cancer cell may have
less than one kind of cause, but there is good reason to               【S7】______
consider the relationships that exist between viruses
and cancer.
    Since there is no evidence which human cancer, as                  【S8】______
generally experience, 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 assume that an innocuous                    【S9】______
latent virus might mutate to form a strain that causes
cancer. Certainly the experiment evidence now available                【S10】______
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.
【S9】
选项
答案∧ assume→to
解析 动词不定式小品词to缺失。it在句中是形式主语，真正的主语是后面的动词不定式 to assume that an innocuous latent virus might mutate to form a strain that causes cancer，因此，assume前面加上“to”。
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