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

Multiple Choice

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) "3 $\pi$ /4"
D) " $\pi$ "
E) "2 $\pi$ "