Early Settlers in the Town of Dry Gulch Drilled Wells

Early settlers in the town of Dry Gulch drilled wells to pump as much water as they wanted from the single aquifer beneath the town.(An aquifer is an underground body of water. ) As more people settled in Dry Gulch,the aquifer level fell and new wells had to be drilled deeper at higher cost.
Refer to the information given above.The aquifer beneath Dry Gulch is:

A) an external cost.
B) private property.
C) a commons.
D) an external benefit.