— Dad, may I go out with my friends? — Unless your homework          .

Mary is tired of learning because she is_____ to do better than she can, both at school and at home. 

These English songs are very popular and they often____ on the radio all the time.

--When _____ the abacus _____?   -- In the sixth century by the Chinese people.

—Mr. Black, may I leave now?—Well, if your work ________, you can go home.