The sickle-cell allele is pleiotropic (i. e., it affects more than one phenotypic trait). Specifically, this allele affects oxygen delivery to tissues and affects one's susceptibility to malaria. Under conditions of low atmospheric oxygen availability, individuals heterzygous for this allele can experience life threatening sickle cell "crises". Such individuals remain less susceptible to malaria. Thus, pleiotropic genes/alleles such as this can help explain why
A) new advantageous alleles do not rise on demand
B) evolution is limited by historical constraints.
C) adaptations are often compromises
D) chance events can affect the evolutionary history of populations.