In a longitudinal mechanical wave, which pair correctly names its alternating regions along the direction of propagation?

• A. Compressions and rarefactions
• B. Electric and magnetic field peaks
• C. Nodes and antinodes
• D. Crests and troughs

Answer: (A)

Verified step by step guidance

1. Understand that a longitudinal mechanical wave consists of particles oscillating parallel to the direction of wave propagation.
2. Identify that in such waves, regions where particles are close together are called compressions, representing areas of high pressure and density.
3. Recognize that regions where particles are spread apart are called rarefactions, representing areas of low pressure and density.
4. Note that terms like crests and troughs are used for transverse waves, where oscillations are perpendicular to the direction of propagation, so they do not apply here.
5. Conclude that the correct pair naming the alternating regions in a longitudinal mechanical wave is 'compressions and rarefactions'.