Why is it possible for an amino acid to be specified by more than one kind of codon?

a)some codons have the same sequence of nucleotides.
b)there are 64 different kinds of codons but only 20 amino acids.
c)some codons do not specify an amino acid.
d)the codon aug codes for the amino acid methionine and serves as the start codon for protein synthesis.

Humans are believed to have evolved in coastal regions in east africa. the region had an abundant supply of fish for early humanoids to eat. when scientists analyze the fads gene they see an interesting pattern. people whose families have lived in this area of east africa for generation show a high level of diversity in alleles for the fads gene. conversely, people whose families had migrated inland a moderate distance from sources of fish showed a much lower diversity for fads gene alleles. additionally, the fads alleles found in people whose family has lived inland for generation are almost all gene alleles which produce fads proteins with a high level of function and activity. how do anthropologists explain this?