Amino acids usually exist in the form of Zwitter ions. This means that they consist of:

• A
  The basic group $-NH_2$ and the acidic group $-COOH$
• B
  The basic group $-NH_3^+$ and the acidic group $-CO_2^-$
• C
  The basic group $-CO_2^-$ and the acidic group $-NH_3^+$
• D
  No acidic or basic group