What is the reason for the prevalence of the sickle-cell allele in African populations?
A. Sickled cells help prevent vitamin A deficiency among African populations.
B. Sickled cells are better able to clot blood.
C. African populations tend to be at higher altitudes, and sickled cells better bind oxygen in these environments.
D. Heterozygotes have some resistance to the parasite that causes malaria