In Order for the Atoms in a Neon Discharge Tube

Multiple Choice

In order for the atoms in a neon discharge tube (or a neon beer sign) to emit their characteristic orange light, it is necessary that

A) the atoms be continually replaced with fresh atoms, because the energy of the atoms tends to be used up with continued excitation, resulting in dimmer and dimmer light.
B) electrons first be given energy to raise them from their ground state to an excited state.
C) there be no unoccupied energy levels in each atom.
D) each atom carry a net electric charge.