Heavy nuclei are unstable because

Heavy nuclei are unstable because
 a. each nucleon is a separate particle that is not acted on by the nuclear force. b. there are not enough protons present relative to the number of neutrons for the electrical force to be strong enough.
  c. the nuclear force dominates the Coulomb repulsive force at distances less than 2 fm, but falls off rapidly at greater distances.
  d. nuclei are stable only when the number of neutrons equals the number of protons.
  e. nuclei are stable only when the number of protons exceeds the number of neutrons.