21. Historically, __________ main material for making tables has been wood, but _________ metal and stone have also been used.
A. the; / B. /; / C. the; the D. /; the
22. ---- I can’t get through to the general manager’s office anyhow.
---- The line is busy. Someone ___________ the telephone.
A. can have used B. can use C. must have used D. must be using
