---What’s wrong with him? He is so sad.---He was ____ at not being invited to the party.
She locked her room _____ a thief would break into it.
She said she would spend her holiday ___ Beijing.
Whenever I go to Beijing, ____ is quite frequent, I will go to visit the Great Wall.
The two boys fought and fought until one ______.
He will _____ our class to sing at the school singing contest.