In the hypothetical population of tree swallows, 18 individuals are homozygous for the c4 allele, 22 individuals are heterozygous for the allele, and 10 individuals lack the allele. What is the frequency of the c4 allele?
a. 0.29
b. 0.36
c. 0.40
d. 0.58
e. 0.80