21. The nurse tried to make ______ difference by working in some poor areas which were in _____mess. 

A. / ,  a                      B. a , /                        C . the, a                D. a, a

22. You can’t correct your mistakes_____ you have the ____to admit it.

A. if, bravery           B. when, strength        C. unless, courage        D. as, power

23. ---This is the seat for the disabled.   --- Oh, sorry. I ______that.

A. didn’t realize        B. don’t realize        C. haven’t realized        D. hadn’t realized