A particular type of virus occasionally undergoes antigenic shift. In which of the following situations is antigenic shift most likely?

A) when RNA polymerase makes a mistake, introducing an incorrect nucleotide
B) when the proofreading ability of RNA polymerase does not function properly
C) when a particular host cell is infected by a single strain of the virus
D) when a particular host cell is infected by two strains of the virus at the same time
E) when the proportion of immune individuals in the population is low