21. --- Have you seen __________ pen ? I left it here this morning .

   --- Sorry , I haven't .

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

22. He often goes swimming __________ Saturday afternoons .

  A. in                        B. at                                   C. on                           D. of

23. This is __________ iPhone . Yours is over there .

  A. she                       B. her                          C. hers                         D. herself