<prompt> Antigenic variation is seen in all EXCEPT:Influenza type AInfluenza type BInfluenza type CNone of the above  <response>: ANSWER: C Influenza type CREF: Park 18th ed Page 144INFLUENZA VIRUS:Family orthomyxoviridaeIncubation period: 18-72 hoursTypes: A, B, C No cross immunity and antigenically distinctBoth A and B have surface antigens, H hemagglutinin, N neuraminidaseMajor antigenic variations occur in A, some in B while C seems to be antigenically stable.Rimantidine and Amantadine are used as prophylaxis for influenza AAntigenic variations:Antigenic shift: sudden change , due to genetic reassortment, causes major pandemicsAntigenic drift: A gradual change, due to point mutations, causes epidemics. 