Which of the following statements best describes the charges on amino acids at neutral pH (around 7)?
A. Each amino acid contains a positively charged amine and a negatively charged carboxylic acid at neutral pH.
B. For some amino acids, the amine is positively charged and the carboxylic acid is uncharged at neutral pH.
C. For some amino acids, the amine is uncharged and the carboxylic acid is charged at neutral pH.
D. For each amino acids, the a