Two Beams of Coherent Light Start Out at the Same

Multiple Choice

Two beams of coherent light start out at the same point in phase and travel different paths to arrive at point P. If the maximum constructive interference is to occur at point P, the two beams
Must travel paths that differ by

A) a whole number of wavelengths.
B) a whole number of half-wavelengths.
C) an odd number of half-wavelengths.