GRE Logical Reading: ETS-GRE逻辑单题ETS - V519X8789PS0THK1K$

Pollutants in the atmosphere can cause acid rain (rain with high acidity levels). While acid rain in itself cannot significantly affect the acidity of bodies of water into which it falls, it can greatly increase the acidity of nearby lakes by increasing the amount of decaying matter on a forest floor. A recent increase in the acidity of the water in Forest Lake, therefore, surely indicates that the rain falling nearby has become more acid.