Why is the one-letter code 'A' assigned to Alanine despite not being unique?

• A. Alanine is the smallest amino acid.
• B. Alanine is the most hydrophobic amino acid.
• C. Alanine is more commonly found in proteins than other amino acids starting with 'A'.
• D. Alanine was discovered before other amino acids starting with 'A'.

Answer: (C)