第12题: [单项选择]Which of the following contributes to the low-cost of using rainwater A. Rainwater is free of chemical treatments. B. People have been harvesting rainwater for centuries. C. The water harvesting system is built on a platform. D. Rainwater can be collected using pipes. 参考答案:A 答案解析:第二段第三句Unlike ground water,rainwater contains no minerals or salts and is free of chemical treatments