The electron in a hydrogen atom makes a transition from an excited state to the ground state. Which of the following statements is true?
a. The transition emits a photon with energy equal to the energy difference between the excited state and the ground state.
b. The transition absorbs a photon with energy equal to the energy difference between the excited state and the ground state.
c. The transition results in the electron moving to a higher energy level.
d. The transition results in the electron moving to a lower energy level.