The best answer is (d) water from snowmelts. The American Southwest has many mountains that catch moist air moving westward (from the Pacific) that falls as snow during the winter. The snow melts gradually during the spring and summer when there is little precipitation. The melted snow runs off the mountains into rivers and streams that can be tapped by irrigation pumps. The snowmelt also sinks into the ground, replenishing underground aquifers that can be tapped by wells in the valleys where farming takes place.
