Question

the descent of masses of snow from their original positions sometimes happens suddenly in avalanches, but it is usually very gradual in the form of glaciers. also, snow exposed to warmer air ultimately melts and flows away as water. which of the following sentences most likely precedes this excerpt? some of these natural occurrences reduce the amount of snow on the mountainside. some snow always stays on the mountainside. once snow slopes become too steep, the lower layers can no longer remain on the mountainside. each time it snows, more snow accumulates on the mountainside.

Explanation:

The passage describes how snow can leave its original position (avalanches, glaciers) or melt away. The most logical preceding sentence would introduce the idea that some snow stays on the mountainside, creating a contrast to the ways snow can be removed or displaced.

Answer:

Some snow always stays on the mountainside.