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A. Measles generates a weak immune response that allows reinfection of hosts, and therefore does not easily evolve escape variants.
b. Human influenza a virus evolves escape variants, resulting in a distinctive cactus-shaped phylogeny.
c. Measles experiences strong selection for escape variants, resulting in a phylogeny similar to a nonpathogenic species.
d. Human influenza a generates lifelong immunity, so there is no selection for escape variants.

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The correct answers are:

b. Human influenza a virus evolves escape variants, resulting in a distinctive cactus-shaped phylogeny.  

All types of Human influenza viruses evolve to escape immunity induced by prior infections and vaccinations.

c. Measles experiences strong selection for escape variants, resulting in a phylogeny similar to a nonpathogenic species.  

As a result of measles infection, specific antibody and CD4 and CD8 T cell responses are generated and contribute to virus clearance and protection from reinfection.