At the turn of the 20th century, Dutch physician Christiaan Eijkman showed

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问题 At the turn of the 20th century, Dutch physician Christiaan
Eijkman showed that disease can be caused not only by
microorganisms but by a dietary efficiency of certain substances【M1】______
now called vitamins. In 1909 German bacteriologist Paul Ehrlich
introduced the world’s first bactericide, a chemical designed to kill
specific kinds of bacteria with killing the patient’s cells as well.【M2】______
Following the discovery of penicillin in 1928 by British
bacteriologist Sir Alexander Fleming, antibiotics joined medicine’s
chemical armory, making the fight against bacterial infection
almost a routine matter. Antibiotics cannot act as viruses, but【M3】______
vaccines have been used to greatly effect to prevent some of the【M4】______
deadliest viral diseases. Smallpox, once a worldwide killer, was
completely eradicated by the late 1970s, and in the United States a【M5】______
number of polio cases dropped from 38,000 in the 1950s to less
than 10 a year by the 21st century.
By the middle of the 20th century scientists believed they
were well on the way to treating, preventing, or eradicating many
of the most deadly infectious diseases that plagued humankind for【M6】______
centuries. And by the 1980s the medical community’s confidence【M7】______
in its ability to control the infectious diseases had been shaken by【M8】______
the emergency of new types of disease-causing microorganisms.【M9】______
New cases of tuberculosis developed, caused by bacteria strains
that were resistant to antibiotics. New, deadly infections on which【M10】______
there was no known cure also appeared, including the viruses that
cause hemorrhagic fever and the human immunodeficiency virus
(HTV), the cause of acquired immunodeficiency syndrome. [br] 【M6】

选项

答案 ∧plagued一had

解析语法错误。该句的谓语动词是believed，属于过去时，that引导的是定语从句，修饰其先行词infectious diseases．意思为“已经感染人类几个世纪的传染性疾病”，此动作是发生在主句动作befieved之前，所以要将时态改为过去完成时。

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