21. We can never expect    bluer sky unless we create     less polluted world.
A. a; a                        B. a; the
C. the; a                       D. the; the
22. - I wonder how much you charge for your services.
-- The first two are free       the third costs $30.
A. while                        B. until
C. when                        D. before
23. Daniel\'s family      their holiday in Huangshan this time next week.