<prompt> Not true regarding influenza virus is -a All types exhibit antigenic shiftb Only type A shows antigenic driftc Drift is accumulation of point mutationsd None of the aboveacaabbc  <response>: Antigenic shift is seen only in Influenza A virus.Antigenic drift is seen in Influenza A and B.Antigenic drift involves point mutation in the gene owing to selection pressure by immunity in the host population.Influenza shows cyclic trend due to these antigenic variationsType A epidemics every 2-3 yearsType B epidemics every 4-7 yearsType A pandemics every 10-15 years 