the water in the river is much better than （ ）in the lake.选A这边前后是指两条河里的水的比较,所以在后面一句中要加上that指代the tiver再举个例子：The weather in Shanghai is waarmer than that in Beijing...为什么不能用it这样的话两个指代的东西就不一样了，前面是河里的水，后面就只是水，加了that后就特指湖里面的水这是that的特殊用法，记住就可以了