单项选择题

According to the passage , before microbes were discovered it was believed that some diseases were caused by ______.

A．germ-carrying insects
B．certain strains of bacteria
C．foul odors released from swamps
D．slimy creatures living near swamps
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单项选择题
It can be inferred from the passage that ______ is not the component of a virus.
A．plant cells
B．RNA
C．carbohydrates
D．a coat of protein
单项选择题
The author implies that bacteria were investigated earlier than viruses because ______.
A．viruses are found only in hot climates
B．viruses are extremely poisonous
C．bacteria are harder to eradicate
D．bacteria are easier to detect
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