21.I would have told him the answer had it been possible, but I _____ so busy then.
A. had been B. were C. was D. would be
22. Your smile is one of the strongest tools that make ______ possible for you to meet new people.
A. / B. them C. this D. it
23. With my money _______, I went back home.
A. ran out of B. ran out C. running out of D. running out
