One of them hasn't prepared __ lessons very well.
Christmas is a holiday usually celebrated on December 25th ____________ the birth of Jesus Christ.
The foreign guests, ____ were government officials, were warmly welcomed at the airport.
I won’t call you ________ something unexpected happens.
I don’t know ________ is in charge of English department.
---Susan is so excited!---Oh, yes! She is busy preparing for her trip to Africa that she _____ for years.