The Margins of Glaciers Are Usually Darker Than the Interior

Multiple Choice

The margins of glaciers are usually darker than the interior primarily because

A) shearing causes the ice to expel air and become darker.
B) glaciers are transparent so the underlying bedrock is visible.
C) the sides are moving faster than the interior of the glacier.
D) the sides have a higher concentration of rocks and other debris.
E) All of these.