Why Are Many Human Recessive Disease Alleles Maintained in Populations

Why are many human recessive disease alleles maintained in populations despite continuing selection against them?

A) Heterozygotes have a higher fitness than either homozygote.
B) Mutations convert recessive alleles to dominant alleles.
C) Even when the allele frequency is low, many gametes contain the recessive allele.
D) Allele frequency remains constant from generation to generation.