Identify the main text structure used in the passage below.
A river is a large natural stream of flowing water. The source of a river is the
place where the river starts. This is usually the highest point in the river. Many
rivers begin in springs, places where underground water flows to the Earth's
surface. Some rivers begin in melting glaciers high up in the mountains or in
lakes.

A. sequence
B. description
C. cause and effect
D. problem and solution