it 和 thatThe waterin the lake is cleaner than ( ) in the riv

1.it代替「全同的」一个事物,例如：
There is a watch on the table; it is mine.
而they代替「全同的」复数人事物,例如：
There are two boys playing on the playground; they are Mary's brothers.
2.that 代替「同类的」单数人事物,避免同一句内名词重复,例如：
The nose of the elephant is longer than that of the cat.
上句的that 代替the nose.你的问题中,that 代替the water.
而those代替「同类的」复数人事物,避免同一句内名词重复,例如：
The ears of the elephant are bigger than those of the cat.
上句的those代替the ears.
3.it或they后面不能接限定片语,故it in the water 无意义.
4.one及ones也可代替重复名词,但不同以上所述,请自行研究.

B
that是代词`代指 water.

B
that是代名词，指“那边”的水。

B

that 指water（英语的习惯）,复数用these代替