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 of viruse
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 of viruses recombine
D. two viruses of the same type are contracted