You are presented with the question "What are formed by flowing rivers through erosion over long periods of time?" and the following answer choices: 
- deserts
 - a hydraulic system
 - deep valleys
 - pyramids
 - Calcium carbonate
 - patch reefs
 - buildings
 - Sediment

Now knowing that most canyons are formed by flowing rivers through erosion over long periods of time and deep canyons and valleys are carved out, choose the best answer.


 

   deep valleys      