The Exact Mass of a ¹²C Atom Does Not Equal

The exact mass of a ¹²C atom does not equal the sum of the mass of 6 protons, 6 neutrons, and 6 electrons because

A) a ¹²C atom has 7 neutrons, not 6.
B) some of the mass of the protons and neutrons is converted to energy when they are combined to form the ¹²C atom.
C) carbon occurs naturally as a mixture of isotopes with slightly different masses.
D) the enthalpy of atom combination of ¹²C is negative.
E) none of the above