根据句意，用所给单词或短语的适当形式填空。

population, pollute, and so on, because of, in need

Water         has made rivers dirty
Which country has the largest           _in the world.
Most people are sick            the cold weather.
—What sports do you like?
—I like playing basketball, playing table tennis, playing football, running _____         ____         .
A friend _         __is a friend in deed.