15.A Michelson interferometer is used to measure the index of refraction of some unknown gas as shown in the right figure. Laser light at 600 nm is shined on a beam splitter and is reflected by two mirrors. The cell with thickness d = 5 cm is initially pumped to vacuum pressure, and the gas is slowly filled to atmospheric pressure. During this process, 100 bright-dark-bright fringe shifts are observed by the detector. What is the index of refraction of the gas at atmospheric pressure at 600 nm?

 
(A) 1.00024
(B) 1.0003
(C) 1.0006
(D) 1.0012
(E) 1.003