The Phase Difference Between the Two Waves Which Give Rise $\pi$

Multiple Choice

The phase difference between the two waves which give rise to a dark spot in a Young's double-slit experiment is (where m = integer) :

A) zero
B) 2 $\pi$ m + $\pi$ /8
C) 2 $\pi$ m + $\pi$ /4
D) 2 $\pi$ m + $\pi$ /2
E) 2 $\pi$ m + $\pi$