Which sentence best explains how the R group (side chain) determines an amino acid's unique chemical behavior?

• A. The R group differs among amino acids and confers properties such as size, charge, polarity, and ability to form special bonds, thereby influencing local interactions and overall protein structure.
• B. The R group always donates a proton, making all amino acids acidic and indistinguishable in terms of folding tendencies and interactions.
• C. The R group contains the peptide bond that links adjacent amino acids and therefore determines whether amino acids can be incorporated into proteins.
• D. The R group is identical for all amino acids except glycine, so primary sequence is the only determinant of tertiary structure.

Answer: (A)