According to the model by Case and Taper, gene flow plays a role in limiting species ranges because
a. Alleles moving into the marginal populations are adaptive for the conditions at the margins of the species range
b. Species in the edges of the range are hybridizing, and hybrids between species have lower fitness
c. Gene flow from populations at the margin of a species range to populations in the center reduces the size of populations in the margins
d. Alleles moving into the marginal populations are adaptive for the conditions at the center of the species range
e. gene flow reduces genetic variation in marginal populations