question_answer 23)

  A set of atoms in an excited state decays.                      (a) in general to any of the states with lower energy (b) into a lower state only when excited by an external electric field (c) all together simultaneously into a lower  state                    (d) to emit photons only when they collide           

Answer:

  A set of atoms in an excited state decays in general to any of the states with lower energy. Choice (a) is correct.