Definition: You are given a question on college physics. You are also given 4 answer options (associated with "A", "B", "C", "D"), out of which only one is correct. You need to answer the question by selecting the correct option. You should only answer with the choice letter, not the whole answer.
Input: Which of the following lasers utilizes transitions that involve the energy levels of free atoms?
(A)Diode laser (B)Dye laser (C)Free-electron laser (D)Gas laser

D