Smallpox Is an Enveloped Virus with a DNA Genome

Smallpox is an enveloped virus with a DNA genome.We have eliminated this once deadly virus through an aggressive worldwide vaccination program.Why might a vaccine be successful with smallpox,but not with HIV?

A) Host cells have RNA in them, and thus do not raise antibodies against RNA.
B) Viruses with DNA genomes must replicate inside a host cell.
C) Smallpox is an enveloped virus.
D) RNA is more stable than DNA, and is less likely to be destroyed by the vaccine.
E) DNA genomes mutate less rapidly than RNA genomes.