Sickle Cell Anemia Evolved in Africa $\beta$ -Globin Has Reached High Frequencies Because Individuals with One Copy

Multiple Choice

Sickle cell anemia evolved in Africa.Although homozygotes of this allele have the anemia,the sickle cell allele of $\beta$ -globin has reached high frequencies because individuals with one copy of the allele have a resistance to malaria.The maintenance of both the normal and sickle cell alleles is therefore due to

A) the Hardy-Weinberg principle.
B) stabilizing selection.
C) Muller's ratchet.
D) directional selection.
E) heterozygote advantage.