Stream Valleys Have a Distinctive V- Shape Because

Stream valleys have a distinctive V- shape because:

A) as a valley is cut, overland flow and mass movement remove loosened material from its slopes.
B) as a stream erodes downward, it encounters material more resistant to lateral erosion.
C) as a stream erodes downward, it becomes graded and accomplishes less lateral erosion.
D) as a valley becomes established, there is less lateral meandering.