Two beam of light having intensities $I$ and $4I$ interfere to produce a fringe pattern on a screen . The phase difference between beams is $\large\frac{\pi}{2}$ at point A and n at point B. Then the difference between resultant intensities at A and B is

Question

$(a)\;2I \\ (b)\;4I \\ (c)\;5I \\ (d)\;7I$