Influenza Strains That Sweep Around the World Often Carry Names

Influenza strains that sweep around the world often carry names such as Shanghai H₁N₁ or Mexico City H₂N₂. The viruses vary in H and N surface proteins because

A) the viruses reproduce and attack people in cities more often.
B) these viruses emerged as stray DNA from the genomes of people in these cities.
C) this is where the antibodies of immune people began to break down and the old virus was again virulent.
D) when infected people develop immunity to the present virus, strains that mutate sufficiently to be outside the range of immunity are soon spread in highly populated areas.