Which statement describes how amino acids combine to form the backbone structure of proteins?

a. they form peptide bonds between the amino group of one amino acid and the carboxyl group of another amino acid.
b. they form peptide bonds between the amino group of one amino acid and the amino group of another amino acid.
c. they form peptide bonds between the amino group of one amino acid and the r group of another amino acid.
d. they form peptide bonds between the r group of one amino acid and the r group of another amino acid.