（ ）21. It was very important for us ________ happy every day.
A. to stay B. stay C. stayed D. staying
（ ）22. I don’t feel like ______ anything now, for I am so tired.
A. does B. to do C. did D. doing
（ ）23 It is no use _______ about the coming exam.
A. worry B worrying C worried D to worry
