<prompt> Cause of antigenic drift in influenza viral infections isSmall mutation in neuraminidase and hemagglutininLarge mutations in hemagglutinin onlyStep mutations in viral genomeNone of the above  <response>: antigenic drift - small changes in the genes of influenza viruses that happen continually over time as the virus replicates.Antigenic shift - an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin and neuraminidase proteins in influenza viruses that infect humans. 