Answer:
Unequal Wavelengths
Explanation:
"To set up a stable and clear interference pattern, two conditions must be met: The sources of the waves must be coherent, which means they emit identical waves with a constant phase difference. The waves should be monochromatic - they should be of a single wavelength." So unequal wavelengths will limit the interference between the waves.
(I put this answer on the quiz and got it right also)
