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目前西安地铁项目正式通过国家批准；
Now, the metro project of Xi'an has been officially approved by the state.
直接产出中的后两类和间接产出都属于地铁项目的正外部性。
So the later two kinds of direct outputs and the indirect profits are the positive externalities of metro projects.
地铁项目在全寿命周期内存在大量的不确定性风险因素，阻碍了项目建设目标的实现。
There are many risks in metro engineering during its life cycle, which are the serious barriers to realize the settled goals.
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