In a Newton's Rings Apparatus, the Interference Effect Observed Between

In a Newton's rings apparatus, the interference effect observed between the rays occurs because of the path difference between

A) the top and bottom surfaces of the top piece of glass.
B) the top and bottom surfaces of the bottom piece of glass.
C) the top and bottom surfaces of the air between the pieces of glass.
D) the top and bottom surfaces of the oil between the pieces of glass.
E) the top surfaces between the top and bottom pieces of glass.