Final answer:
For complete destructive interference between two waves, their crests must align with the troughs of the other wave. In other words, the peaks and valleys of the two waves must be in exact phase opposition with each other.
Step-by-step explanation:
For complete destructive interference between two waves, their crests must align with the troughs of the other wave.
In other words, the peaks and valleys of the two waves must be in exact phase opposition with each other.
When this happens, the disturbances caused by the waves cancel each other out, resulting in zero amplitude, or complete cancellation of the waves.