P and Q are two small loud speakers which emit sound waves of the same amplitude but with a phase difference of π. A small receiver R moves along the perpendicular bisector of PQ in the direction away from P and Q. The intensity of the sound recorded in the receiver is :
(a) continuously decreasing tending to zero at a very large distance
(b) alternates between a constant maximum and zero minimum
(c) alternates between diminishing maximum and increasing minimum
(d) *** constant equal to zero.
