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13、_____ over many years,the underground system is now able to transport millions of people effectively.

A.Being developed                                       B.Developing

C.To develop                                            D.Having been developed

13、D

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_____ over many years,the underground system is now able to transport millions of people effectively.

A.Being developed                                       B.Developing

C.To develop                                            D.Having been developed

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