Final answer:
Amplitude in a longitudinal wave is related to the maximum displacement of particles from their equilibrium position and is not measured by the distance between high-pressure areas like compressions, so none of the provided options correctly describes how to measure amplitude in longitudinal waves.
Step-by-step explanation:
In a longitudinal wave, the amplitude cannot be measured by the distance between high-pressure areas because those areas correspond to compressions rather than crests or troughs. Amplitude, in the context of longitudinal waves, refers to the maximum displacement of particles from their equilibrium position, which is not measured in the same way as it is for transverse waves. In transverse waves, crests are the highest points the medium rises to and troughs are the lowest points it sinks to, and the amplitude is the distance from the equilibrium position to either the crest or trough. However, for longitudinal waves, the amplitude is typically related to the density or the pressure difference in the medium, since the wave travels through compressions and rarefactions rather than crests and troughs. Therefore, the correct answer to the question is not provided in the options, as amplitude in a longitudinal wave is not measured from high-pressure areas, but is typically considered to be the maximum extent of particle displacement from the rest position in the medium.