If the Condition Known as Sickle Cell Anemia Involves a Mutant

If the condition known as sickle cell anemia involves a mutant variant of a gene that can be fatal, why does it have a frequency as high as 20 percent in the gene pool of certain populations?

A) It is theorized that in areas where the malarial parasite is endemic, the advent of agricultural practices leads to a change in natural selection pressures acting on the genetic traits of these populations.
B) People bitten by an infected mosquito have a better chance of resisting the malarial parasite if they carry one copy of the mutant variant in addition to the normal hemoglobin variant.
C) Because the combined effects on a population of more than one disease can be exacerbated by a variety of stressful environmental factors.
D) Both a and b
E) None of the above.