单项选择题

What is known about the influenza virus

A．It was first found in a group of very old birds.
B．All the different strains can be found in wild birds.
C．It existed over 100 million years ago.
D．It can survive in many different places.

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热门试题

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

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