单项选择题

According to the passage, scientists have discovered that influenza viruses ______ .

A．cause ill health in wild birds
B．do not always cause symptoms in birds
C．are rarely present in wild birds
D．change when transferred from animals to man

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热门试题

单项选择题

According to the passage a great variety of influenza can appear when ______ .

A．H and N spikes are produced
B．animal and bird viruses are combined
C．dissimilar types of viruses recombine
D．two viruses of the same type are contracted

单项选择题

New strains of viruses are transmitted to man by ______ .

A．A type of wild pig
B．diseased lower animals
C．a group of migrating birds
D．a variety of means