In 1861 a Green Line Was Observed in the Spectrum

In 1861 a green line was observed in the spectrum of selenium ore that led to the discovery of the element thallium.Which of the following was responsible for the green line in the spectrum?

A) excited electrons jumping to a higher energy level
B) excited electrons dropping to a lower energy level
C) excited protons jumping to a higher energy level
D) excited protons dropping to a lower energy level
E) none of the above