Unlike Many Other Genetic Disorders, Huntington's Disease Is Caused by a Full

Unlike many other genetic disorders, Huntington's disease is caused by a full penetrance disease allele. Full penetrance means what?

A) The longer the allele (more triplet repeats) , the more likely the disease is to occur in an individual.
B) Individuals with a disease allele are almost certain to get Huntington's disease.
C) Heterozygotes are just as likely to get Huntington's disease as homozygotes.
D) There are environmental factors beyond age that are also required for an individual to get Huntington's disease.
E) There are other genes that are also required for an individual to get Huntington's disease.