Light nuclei such as hydrogen isotopes release energy on fusion, whereas very heavy nuclei do not. Which statement best accounts for this observation?
(a) The binding energy per nucleon of the fusion product is lower than that of the light reactants, so the excess appears as energy.
(b) Light nuclei experience almost no Coulomb repulsion, so their electrostatic potential energy is released on fusion, whereas heavy nuclei repel too strongly to fuse.
(c) The binding energy per nucleon of the fusion product is higher than that of the light reactants.
(d) Light nuclei have negative mass defects that turn positive after fusion, converting mass into energy; heavy nuclei already have zero mass defect.