Fact 1: Avalley is formed by a river flowing.

Fact 2: Fast flowing rivers tend to pick up more pebbles, sand and other materials that cause erosion.

Given the two facts above, answer the question "How does water cause a valley to form?" with the following options: 
- Physical weathering
 - By the process of erosion
 - By falling from clouds
 - It used to be covered by water
 - fissures of hot, acidic water
 - Heavy snow accumulation
 - By forming puddles
 - Water expanding


 

  By the process of erosion       