The Most Reasonable Explanation for the Fact That Disorders Such

The most reasonable explanation for the fact that disorders such as cystic fibrosis do not disappear from the human population is that

A) it is an autosomal dominant disease.
B) new medical procedures allow most sufferers to live long enough to pass the gene on to offspring.
C) heterozygous carriers have one normal gene that masks the harmful gene.
D) the gene keeps mutating back to the harmful form.
E) persons with cystic fibrosis are made stronger by their affliction.