The only language （ ） is easy to learn is the mother tongue.
（A）which
（B）that
（C）/
（D）it
We'll reach the sales targets in a month （ ） we set at the beginning of the year.
（A）which
（B）where
（C）when
（D）what
He studied hard at school when he was young, （ ） contributes to his success in later life. 选项：
（A）as
（B）therefore
（C）which
（D）so that
