A large population of land turtles on an isolated island has two alleles for a gene that determines shell thickness. The allele for thinner shells is dominant over the allele for thicker shells. The thinner-shell allele occurs at a frequency of 20%. Assuming there is no net advantage to thick or thin shells, what should the frequency of homozygous thicker-shelled turtles be in the population?a. 4% b. 16% c. 50% d. 64%