If 1,2 and 3 represent different mediums and 'n' is refractive index, then which of the following equations is true?

If 1,2 and 3 represent different mediums and 'n' is refractive index,

1.	If 1,2 and 3 represent different mediums and 'n' is refractive index, then which of the following equations is true?
A.	n23 = n31 × n12
B.	n32 = n31 × n12
C.	n32 = n13 × n12
D.	n23 = n13 × n12
Answer» C. n32 = n13 × n12