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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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