Thanks _______ your last letter.[ ]
Mom,where is David?—He is working _____ that difficult math problem in his room.[ ]
Man landed ______ the moon in 1969 ______ the first time. [ ]
Do you know the boy________ is sitting next to Peter? — Yes. He is Peter’s friend. They are celebrating his ________ birthday.
Do you have plenty of friends in your school?