Being Heterozygous for the Hemoglobin Gene Variant That Causes the Disease

Being heterozygous for the hemoglobin gene variant that causes the disease sickle-cell anemia can be advantageous in what situation?

A) in areas where malaria is endemic
B) when iron intake is too low in the diet
C) in elite athletes because it slightly decreases oxygen affinity
D) when a person has anemia caused by other factors
E) when a person is at high altitude