After reading the paragraph, select the sentence that identifies the theme. A glacier is a river of ice. It starts as a pack of snow. When the winter snowfall is so great that it does not melt in the summer, it causes the pack to grow. The summer wind layers it with dirt, and it continues to build into a hard body of ice. The weight of the glacier moves it slowly downhill.

The paragraph tells how glaciers are like rivers.
The paragraph tells how a glacier is formed.
The paragraph tells how glaciers are hard.