The Sickle- Cell Allele Produces a Serious Blood Disease in Homozygotes.Why

The sickle- cell allele produces a serious blood disease in homozygotes.Why doesn't natural selection eliminate this allele from all human populations?

A) In populations where endemic malaria is present,heterozygotes have an important advantage: They are resistant to malaria and therefore are more likely to survive and produce offspring that carry the allele.
B) Natural selection is a positive force,so it does not eliminate alleles.
C) Natural selection occurs very slowly,but elimination of the sickle- cell allele is expected to occur soon.
D) Mutations keep bringing the allele back into circulation.