Up to Tuesday's attack, the worst school shooting in the U. S. was in March 1998 in Jonesboro, two boys, aged 11 and 13, shot and killed a teacher and four girls.
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.