Which of the following examples best describes a unique functional property of the carboxyl group?

• A. Its compounds may be structural isomers with different properties.
• B. Two carboxyl groups can bond covalently to help stabilize protein structure.
• C. It is polar as a result of the electronegative oxygen atom drawing electrons toward it.
• D. The covalent bond between oxygen and hydrogen is so polar that hydrogen ions tend to dissociate from oxygen reversibly.