[单选题]
Some of the energy levels of electrons of a mercury atom are as shown.
Electrons and photons are incident on a mercury vapour causing energy level transitions. Emitted photons due to the transition of electrons between energy levels from the vapour have an energy of 4.9 eV.
Which of the following could be incident into mercury vapour?
I. An electron with an energy of 5.1 eV
II. A photon with an energy of 4.9 eV
III. A photon with an energy of 5.1 eV
