Sound propagates through tissue as a series of compressions and rarefactions. Particles in the medium are pushed together during compression and pulled apart during rarefaction.

• A. Compressions and Rarefactions; Compression; Rarefaction
• B. Rarefactions and Reflections; Expansion; Compression
• C. Reflections and Transmissions; Compression; Expansion
• D. Compressions and Transmissions; Compression; Rarefaction

Answer: (A)

Sound travels through tissue as a wave that alternates between regions where particles are squeezed together and regions where they are pulled apart. Those squeezed regions are called compressions, and those pulled-apart regions are called rarefactions. This pairing—compression and rarefaction—matches exactly the description given: particles are pushed together during compression and pulled apart during rarefaction. Other terms in the distractors describe interactions with boundaries (reflections, transmissions) or use nonstandard wording (expansion) for the low-pressure phase, which isn’t the conventional term here. So the option that uses Compressions and Rarefactions with Compression and Rarefaction is the best choice.