A
定从
先行词为clubs
in+which 相当于where
in 不可以加that
Each city has a number of clubs .In the clubs boys and girls can go to play games.
把两句话连接起来可以写成：Each city has a number of clubs in which boys and girls can go to play games.(which 代替的是clubs)
或者是Each city has a number of clubs where boys and girls can go to play games.(where 代替的是in the clubs)