Influenza pandemics occur periodically due to characteristics of the influenza virus. How did these characteristics, in this case of the H1N1 strain, contribute to the spread of the swine flu pandemic in 2009?

A) The H1N1 virus had a mutation that increased the mortality associated with infection.
B) The H1N1 virus had a mutation that increased the basic reproduction number of the virus.
C) The H1N1 virus underwent a significant antigenic shift compared to other circulating strains of influenza.
D) The H1N1 virus acquired a new gene from pigs that resulted in increased virulence.