Influenza Strains That Sweep Around the World Often Carry Names

Influenza strains that sweep around the world often carry names such as H₅N₁ or H₂N₂.If they are both flu viruses,why do they differ in their H and N numbers?

A) The H and N numbers indicate the geographical region in which the virus was discovered.
B) The H and N numbers indicate the order in which the virus was discovered.
C) The H and N numbers indicate the toxicity of the virus to humans and nonhumans.
D) The H and N numbers indicate the type of glycoprotein spikes found in the viral envelope.