单选题
Water on the earth is being continuously recycled 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 second step is the water returning to the earth in the form of precipitation, 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 seeps (渗透) 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% of the earth's water is in the oceans, and is too salty for humans of plants. Of the 5% on land, only 0.05% is above ground in rivers or lakes. The rest is underground water. This groundwater 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 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 an increasing population and more toxic(有毒的)waste, the hydrologic cycle we depend on could become dangerously imbalanced.