______ causes dramatic changes in a flu virus that can allow it to jump species while ______ refers to more gradual changes in a flu virus that can cause issues with the effectiveness of the yearly flu vaccines as well as the severity of symptoms of a particular strain of flu.
a) Antigenic Shift; Antigenic Drift
b) Mutation; Adaptation
c) Genetic Recombination; Viral Variation
d) Transmission; Virulence