Which of the following could excite an electron in a sample of neon gas into a higher energy state?
a. Absorption of a photon with energy equal to the energy difference between the electron's current state and the higher energy state
b. Collision with another high-energy electron
c. Application of a strong external electric field
d. Absorption of a photon with energy lower than the energy difference, followed by absorption of additional photons to make up the difference