单项选择题

It has been discovered that______.

A．H5N1 does not transmit among human beings
B．bird flu spreads only among birds
C．H5N1 is a variant of flu virus
D．bird flu adapts easily to other species

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热门试题

单项选择题

H5N1 is a flu strain ______.

A．limited to Hong Kong
B．which first affected human beings in 1961
C．crossing species barriers
D．which was first isolated from American birds