单项选择
( )21.This is ________Mr.Smith. He works in________office.
A.a,a B.an;an C./;an
( )22.They are in________same school, but in_________different grades.
A.the;/ B.a;the C.the;the
( )23.These are_________and those are_________.
A.knives,box B.knives;boxes C.knife;box
