You Place a Convex Lens on Top of a Flat

You place a convex lens on top of a flat plate of glass and illuminate it with monochromatic light of wavelength 600 nm.You observe a dark circle at the center of the lens,surrounded by a series of concentric dark rings.What is the thickness of the air space between the lens and the flat glass plate where you see the sixth dark ring?

A) 3.90 µm
B) 3.60 µm
C) 1.80 µm
D) 1.95 µm
E) 2.10 µm