Antigenic shift -: (A) Occurs every 2-3 years, (B) Gradual change over time, (C) Results from genetic recombination, (D) Occurs in all influenza viruses
Answer is C. Antigenic shift is an abrupt, drastic, discontinuous variation in the antigenic structure resulting in a novel virus strain unrelated antigenically to predecessor strain. REF: Anantha Narayan AND PANIKER'S TEXTBOOK OF MICROBIOLOGY 9TH EDITION PAGE NO:499