In a Young's Double-Slit Experiment the Center of a Bright $\pi$

In a Young's double-slit experiment the center of a bright fringe occurs wherever waves from the slits differ in phase by a multiple of:

A) $\pi$ /4
B) $\pi$ /2
C) $\pi$
D) 3 $\pi$ /4
E) 2 $\pi$