What Factor Allows the Dominant Mutant Allele for Huntington's Disease

What factor allows the dominant mutant allele for Huntington's disease to persist in human populations despite its devastating effects?

A) Heterozygote advantage.
B) The fact that its symptoms do not typically show until after the individual has had children.
C) Its effects are not expressed in heterozygous individuals.
D) Its effects are only expressed in homozygous recessive individuals.
E) Its effects are only expressed in homozygous dominant individuals.