单项选择题

This passage ______.

A．concerns the damaging effects of influenza
B．mentions the steps of fighting against the harmful effects of influenza
C．emphasizes the worry expressed by all age groups
D．both A and B

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单项选择题

According to the passage, high-risk persons exclude which of the following kinds of people

A．Children suffering from asthma.
B．The elderly with chronic pulmonary diseases.
C．Middle aged people with chronic heart diseases.
D．Nurses taking special care of the sick.

单项选择题

That a different component part of the vaccine is necessary is principally due to the variable change of ______.

A．virus
B．strain
C．antigen
D．immunization