改错。

( ) 1. I can plays the piano.                              ______________

                A     B    C

( ) 2. Jenny plays the erhu three time a week.    ______________

                        A                   B     C

( ) 3. Danny is a bad piano play.                       ______________

                      A      B            C

( ) 4. Let I think.                                             ______________

             A  B   C 

简述补偿贸易与易货贸易相比较有何特点？