An "X" Is Marked on the Bottom of a Glass

Multiple Choice

An "X" is marked on the bottom of a glass container.A microscope is adjusted so that it is focused on the "X." A liquid is now poured into the glass to a depth of 6.00 cm.If the microscope has to be backed up by a distance of 1.50 cm to refocus on the "X," then calculate the refractive index of the liquid.

A) 1.25
B) 4.00
C) 1.33
D) 0.75
E) 3.00