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 world-wide vaccination program.Why might a vaccine be successful with smallpox,but not with HIV?

A) Because smallpox is an enveloped virus
B) Because viruses with DNA genomes must replicate inside a host cell
C) Because RNA is more stable than DNA, and is less likely to be destroyed by the vaccine
D) Because host cells have RNA in them, and thus don't raise antibodies against RNA
E) Because DNA genomes mutate less rapidly than RNA genomes