thin film of oil (no = 1.50)with varying thickness floats on water

thin film of oil (n_o = 1.50)with varying thickness floats on water (n_w = 1.33)When it is illuminated from above by white light, the reflected colors are as shown in Fig. 24–60. In air, the wavelength of yellow light is 580 nm. (a) Why are there no reflected colors at point A? (b) What is the oil’s thickness t at point B?