3. which sentence best describes mass movement?
a. the movement of particles within an atom b. the downhill flow of soil and rock resulting from gravity c. the downhill movement of animals due to heavy snow in their normal home d. the steady pace of air bouncing off a mountain

The answer is B. The downhill flow of soil and rock resulting and rock

Mass movement is the movement of surface material caused by gravity ,
the downhill flow of soil is commonly known as landslide is a type of mass movement, so does the downhill flow of rock resulting from gravity ( commonly known as rockfall)

Bwould be my guess |-/