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

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

A) Natural selection is a positive force, so it does not eliminate alleles.
B) 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.
C) Mutations keep bringing the allele back into circulation.
D) Natural selection occurs very slowly, but elimination of the sickle-cell allele is expected to occur soon.