Two coherent microwave beams of equal amplitude overlap in free space. Which of the following conditions is most likely to produce complete destructive interference at a point where the beams meet?
(a) The path length difference between the beams is an integer multiple of λ.
(b) The beams arrive 90∘ out of phase (phase difference π/2).
(c) The path length difference between the beams is an odd multiple of 2λ.
(d) The beams are polarized in perpendicular directions.