Answer the following question given this paragraph: 

Lactic acid fermentation converts the 3-carbon pyruvate to the 3-carbon lactic acid (C 3 H 6 O 3 ) (see Figure below ) and regenerates NAD + in the process, allowing glycolysis to continue to make ATP in low-oxygen conditions. Since there is a limited supply of NAD + available in any given cell, this electron acceptor must be regenerated to allow ATP production to continue. To achieve this, NADH donates its extra electrons to the pyruvate molecules, regenerating NAD + . Lactic acid is formed by the reduction of pyruvate.


Q: What natural destructive element cuts away at rock forming river valleys?


A:
water