B.compressions
Step-by-step explanation:
This is a characteristics of the "longitudinal waves".
Longitudinal waves are waves where the oscillation of the particles of the medium occurs in a direction parallel to the direction of propagation of the wave (an example of longitudinal waves are sound waves). As a consequence, this oscillation creates some regions where particles are closer together, and other regions where particles are more spread apart. The former are called "compressions", while the latter are called "rarefactions".