Water on the earth is being recycled continously in a process known as the hydrologic cycle.The first step of the cycle is the evaporation of water in the oceans.Evaporation is the process of water turning into vapor,which then forms clouds in the sky.The The second step is the water returning to the earth in the form of precipitation:either rain, snow,or ice.When the water reachs the earth's surface,it run off into the rivers,lakes,and the ocean,where the cycle begins again.
　　
　　Not all water,however,stays on the surface of the earth in the hydrologic cycle.Some of it seeps into the ground through infiltration and collects under the earth's surface as groundwater.This groundwater is extremely important to life on earth,since 95 percent of the earth's water is in the oceans and is too salty for human beings or plants.Of the 5 percent on land,only 0.5 percent is above ground in rivers or lakes .The rest is underground water. This groundwater is plentiful and dependable,because it dosen't depend on seasonal rain or snow.It is the major source of water for many cities.But as the population increases and the need for water also increases,the groundwater in some areas is getting dangerously low.Added to this problem is an increasing amount of pollution that seeps into the groundwater.In the future,with a growing population and more toxic water,the hydrologic cycle we depend on could become dangerously imbalanced.
　　
　6. Clouds are formed from _____.
(A) water vapor
(B) evaporation
(C) the hydrologic cycle
(D) groundwater

　7. Water returns to the earth by _____.
(A) infiltration
(B) pollution
(C) precipitation
(D) evaporation

　8. Groundwater _____.
(A) depends on seasonal rain
(B) comes from toxic waste
(C) is 0.5 percent of all water
(D) collects under the earth

　9. The amount of groundwater is _____.
(A) about 95 percent of all water
(B) less than 5 percent of all water
(C) 0.5 percent of above-ground water
(D) 95 percent of above-ground water

　10. The supply of groundwater is going low because of _____.
(A) conservation
(B) toxic waste
(C) pollution
(D) population increase