Two coherent sources of visible monochromatic light form an interference pattern on a screen. If the relative phase of the source is varied from 0 to 27 at a frequency of 500 hertz, which of the following best describes the effect, if any, on the interference pattern? A. It is unaffected because the frequency of the phase change is very small compared to the frequency of visible light. B. It is unaffected because the frequency of the phase change is an integral multiple of 7. C. It is destroyed except when the phase difference is o to 7. D. It is destroyed for all phase differences because the monochromaticity of the sources is destroyed. E. It is not destroyed but simply shifts positions at a rate too rapid to be detected by the eye.