Initially, the two waves from a double-slit travel equal distances to reach point P on the screen, so that P is a bright central maximum. A transparent sheet of thickness 0.20μm and refractive index 1.25 is then introduced in the path of one of the waves only (the other path remains in air). Light of wavelength 500nm in air is used. After inserting the sheet, point P will
a) remain a bright fringe (maximum)
b) become a dark fringe (minimum)
c) show reduced intensity but not become completely dark
d) show no interference pattern at all