Fact 1: Most canyons are formed by flowing rivers through erosion over long periods of time.

Fact 2: Rivers and glaciers are made of water.

Given the two facts above, answer the question "Flowing water forms most canyons through erosion over what?" with the following options: 
- movement of tectonic plates
 - dry streambeds
 - long periods of time
 - Physical weathering
 - Time and energy
 - short periods of time
 - massive damage
 - calving glaciers


 

   long periods of time      