In proteins, what structural feature primarily differentiates one amino acid from another?

• A. The identity of the side chain (R group) attached to the $C$$\alpha$ carbon
• B. The presence of an amino group $-N_{\mathrm{H}}{2}_{\mathrm{}}$
• C. The peptide bond geometry between adjacent residues
• D. The presence of a carboxyl group $-COOH$

Answer: (A)