Two independent monochromatic sodium lamps cannot produce interference because
A. The frequencies of two sources are different
B. The phase difference between two sources changes with respect to time
C. Two sources become coherent
D. The amplitude of sources is different.

Hint: Interference is the phenomenon of superposition of crest and trough of two waves of same or different wavelengths.

Complete step-by-step answer:
First, we should know about monochromatic and coherent terms.
Monochromatic sources mean those sources which emit only light of single and fixed wavelength.
And, coherent sources are those which have the same phase difference.
The sources which have same phase difference cannot interfere because the crest of one and trough of other and vice versa will overlap each other and cancel out each other,
Therefore, we cannot define the interference pattern of two coherent monochromatic sources.

Additional Information
The principle of interference was discovered by Thomas Young, who did single slit experiment and double slits experiment and observed this effect. Whenever, either of two crests or two troughs superimpose a bright fringe on the other hand, when one crest and one trough superimpose, they cancel out each other and form a dark fringe. This is a continuous pattern and this bright and dark fringe pattern is known as interference pattern.

Note: All the students must be aware of the terms like coherent sources, monochromatic sources and interference to approach and solve the questions logically. Also, the diffraction and interference occur simultaneously but for the simplicity of question these both are treated as differently.