（ ）19．—Is this ______ classroom?
—No, it’s theirs.
A. we B. us C. our D. ours
（ ）20．— What did you do ______ Sunday morning?
—We just stayed at home and watched TV.
A. on B. in C. at D. for
（ ）21．My parents ______ both teachers. They teach in the same school.
A. be B. am C. is D. are
（ ）22．—You’d better go home at once ______ your mother will be worried.
—All right.
A. and B. or C. but D. so