21. Brussels, the capital city of Belgium, is ______ home to the European union, ____organization of European nations.      

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

22. Few singers can play the piano as will as they can sing; Jay Chou, however, is _______.

A. a signal     B. example        C. a sign     D. an exception

23. Experts have found that the disease will lead to a greater increase in the number of sufferers unless ______ seriously when the symptom first appears.

A. treated   B. having treated     C. be treated     D. to be treated