Why Are Reassorted Influenza Viruses Usually More Likely to Cause

Why are reassorted influenza viruses usually more likely to cause a severe worldwide pandemic?

A) They are more immunosuppressive than other influenza viruses.
B) They contain new segments that produce more pathogenic proteins.
C) They can infect different types of cells.
D) They have acquired new glycoproteins for which there is no prior immunity.
E) They have acquired mutations that allow them to evade the immune system.