Question

which part of the wave is labelled with the letter a? options: compression, rarefaction, trough, crest (with an image of a wave showing direction of travel and labels a, b)

Explanation:

In a longitudinal wave (like the one shown with compression and rarefaction), a compression is a region where particles are close together. The label A points to a region with dense particle arrangement, which is a compression. Rarefaction is where particles are spread out, trough and crest are terms for transverse waves (not longitudinal here).

Answer:

A. Compression