The wavelength of sound in air may be determined by using stationary waves. In one experiment, a loudspeaker produces a sound wave of constant frequency which is reflected directly back along its original path by a metal plate approximately 1m away. A microphone connected to a cathode-ray oscilloscope (CRO) is moved between the loudspeaker and plate to identify regions of high sound intensity ('loud' spots) and low sound intensity ('quiet' spots). The wavelength of the sound is determined using the least possible number of measured quantities. Which row shows the quantities that are needed?

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.