Passage 24

  Water on the earth is being continuously recycled in a process known as the hydrologic cycle(水循环). 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 second step is the water returning to the earth in the form of either rain, snow or ice. When the water reaches the earth’s surface, it runs 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 flows slowly into the ground through infiltration(渗入) and collects under the earth’s surface as ground water. This ground water is extremely important to life on earth since 95% of the earth’s water is in the oceans, and is too salty for humans or plants. Of the 5%on land, only 0.05% is above ground in rivers or lakes. The rest is underground water. This ground water is plentiful and dependable, as it doesn’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 ground water in some areas is getting dangerously low. Added to this problem is an increasing amount of pollution that flows into the ground water. In the future, with an increasing population and more poisonous waste, the hydrologic cycle we depend on could become dangerously unbalanced.

1.Clouds are formed from__________.

A. evaporation

B. water vapor

C. outer space

D. the hydrologic cycle

2.What percentage of the total earth’s water does underground water make up?

A. 0.05%                       B. 0.5%

C. 4.95%                       D. 5%

3.Which of the following is true about ground water?

A.It comes from the oceans.

B.It depends on seasonal rain.

C.It is 95% of all water

D.It collects under the earth

4.The supply of ground water is getting low because of _______.

A.conservation

B.poisonous waste

C.pollution

D.population increase

5.This passage mainly discusses________.

A.water conservation

B.the hydrologic cycle

C.underground water

D.polluted ground water