What a foolish mistake it is （ ） you've made!
（A）it
（B）this
（C）that
（D）which
（ ） was Mary who picked up the wallet.
（A）He
（B）She
（C）They
（D）It
I can't find Mr. Smith. Where did you meet him this morning? — It was in the hotel （ ） he stayed.
（A）that
（B）where
（C）which
（D）the one
