21.I think this is ___ useful book, so I will keep ____for ever.
A. an, it B. a, it C. /, them D. the, them
22.You could go to bed earlier ________you won’t be tired at class.
A. until B. if C. though D. so that
23.I _____ play basketball after supper, but now I ____ having a walk.
A. am used to, used to B. used to, am used to
