In the diagram below (not to scale), each of the loudspeakers emits a continuous sound of the same frequency. The whole system is kept in air. A microphone moved along the line \(PQ\) detects a series of maximum and minimum sound intensities. Which of these following actions on its own, will lead to an increase in the distance between the maxima of sound intensity?
(A) Decreasing the frequency of the sound emitted by the loudspeakers
(B) Increasing the frequency of the sound emitted by the loudspeakers
(C) Decreasing the separation of the loudspeakers
(D) Increasing the distance of the loudspeakers from the line \(PQ\).