The central fringe of the interference pattern produced by the light of wavelength 6000 Å is found to shift to the position of 4th dark fringe after a glass sheet of refractive index 1.5 is introduced. The thickness of glass sheet would be
A. `4.8 mu m`
B. `4.2 mu m`
C. `5.4 mu m `
D. `3.0 mu m `