According to the passage，a great variety of influenza strains can appear when______.
- A H and N spikes are produced
- B animal and bird viruses are combined
- C dissimilar types virus recombine
- D two viruses of the same type are contracted
问题
According to the passage，a great varie
参考解析：
[解析] 见第二段第二句话“The result of such recombinations is a great variety of strains constraining different H and N spikes. ”。
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