Question

unlike a transverse wave, a longitudinal wave travels in a direction that is parallel to the direction the particles in the medium move. when eli moves his hand back and forth, the coils in the spring toy squeeze together and pull apart horizontally. a wave of compressing and stretching then travels along the horizontal length of the spring toy.
complete the sentences.
the longitudinal wave that eli creates in the spring toy travels from left to right and the particles in the spring move left and right. this means that the wave and the particles of matter move in directions.
perpendicular
parallel

Explanation:

The problem is about the properties of a longitudinal wave. As stated in the text, a longitudinal wave travels in a direction parallel to the direction the particles in the medium move. Eli's wave in the spring toy travels from left to right (given), and the particles move left and right (given). Since the wave direction (left - to - right) and particle movement (left - right) are along the same line (horizontal), they are parallel.

Answer:

parallel