A stationary sound wave is set up between a loudspeaker and a wall. A microphone is connected to a cathode-ray oscilloscope (CRO) and is moved along a line directly between the loudspeaker and the wall. The amplitude of the trace on the CRO rises to a maximum at a position X, falls to a minimum and then rises once again to a maximum at a position Y. The distance between X and Y is 33 cm. The speed of sound in air is 330 m s⁻¹. Which diagram could represent the CRO trace of the sound received at X? [Figure 23.1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.