The Most Effective and Cheapest Method of Protecting Groundwater Is

The most effective and cheapest method of protecting groundwater is to

A) pump it to the surface, clean it, and return it to the aquifer.
B) inject microorganisms to clean up contamination.
C) pump nanoparticles of inorganic compounds to remove pollutants.
D) use methods to prevent contamination.
E) none of these answers