结构推理 The world is said to have about 3,000 times as much groundwater as water it has in rivers and lakes, and groundwater is far cleaner.
【正确答案】据说全球拥有的地下水量大约是其拥有的河流和湖泊水量的3000倍，而且地下水要干净得多。
【答案解析】