12. A (n) _ is an underground reservoir of water that is over-

lain by impermeable rock units.

a. aquiclude

c. confined aquifer

b. aquifer

d. unconfined aquifer

b. Aquifer

Explanation:

An aquifer is a body of saturated rock through which water can move quickly. An aquifer must be both penetrable and porous. It includes such rock types as sandstone, conglomerate, fractured limestone, and unconsolidated gravel and sand. Fractured volcanic rocks, such as columnar basalts, can also make suitable aquifers. The rubble zones between volcanic flows are generally both porous and permeable and make excellent aquifers. Rocks such as granite and schist are generally poor aquifers because they have very low porosity.