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21. ---Who will be ____ duty tomorrow?                                                      
   ---Susan will.
   A. at         B. on          C. for          D. in
22. Lao She is the____________of  Tea House(²è¹Ý).                                             
  A. doctor      B. actor       C. scientist       D. writer
23. The teacher ____ his students to hand in their compositions before Friday.