21. There\'s _________800-metre-long road behind _________hospital.  
¡¡ A.an,an                B.a, a                  C.an, the                D.a, the
22. The population of the world ____ still ____ now.
A.has; grown         B.is; growing         C.will; grow            D.is; grown
23. - Excuse me!
- _____
- How can I get to the nearest post office?
A.Yes?                       B.That’s OK.         C.What’s wrong?             D.Pardon?