A General Condition That Two Waves Undergo Constructive Interference Is $\pi$

Multiple Choice

A general condition that two waves undergo constructive interference is that

A) their phase difference is zero.
B) their phase difference is $\pi$ /2 rad.
C) their phase difference is $\pm\pi$ /2 rad.
D) their phase difference is an even integral multiple of $\pi$ rad.
E) their phase difference is an odd integral multiple of $\pi$ rad.