When two nuclei of deuterium (A=2) combine to form one nucleus of helium-4, energy is liberated mainly because
(a) some nucleons are converted completely into energy creating a mass defect
(b) the product has higher binding energy per nucleon than the reactants
(c) the product has lower binding energy per nucleon than the reactants
(d) fusion reduces the electrostatic potential energy of the system, releasing the excess as radiation