Use the Following Information to Answer the Question Below

Multiple Choice

Use the following information to answer the question below.
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20 percent of the β hemoglobin alleles in the human gene pool.
The sickle-cell allele is pleiotropic (that is, it affects more than one phenotypic trait) . Specifically, this allele affects oxygen delivery to tissues and affects one's susceptibility to malaria. Heterozygous individuals who have one normal and one sickle-cell allele have a phenotype that has both costs and benefits. The cost: Under conditions of low atmospheric oxygen availability, individuals heterozygous for this allele can experience life-threatening sickle-cell "crises." The benefit: Such individuals are less susceptible to malaria. Thus, pleiotropic genes/alleles reveal that ________.

A) new advantageous alleles do not arise on demand
B) evolution is limited by historical constraints
C) adaptations are often compromises
D) chance events can affect the evolutionary history of populations