I bought ______ useful dictionary yesterday．_______dictionary is very cheap．（　　）

A．a； AB．a； TheC．an； The

Is this your son's sweater？

No，______ is on the chair behind the desk．（　　）

A．hersB．yoursC．his

Which of the two subjects do you like，P．E．or music？

_______．They are really interesting．（　　）

A．NeitherB．BothC．None

I didn't realize he was a famous scientist ______ you told me．（　　）

A．untilB．becauseC．since

It's necessary for you _______the classroom clean，class．（　　）

A．keepB．keepingC．to keep