A government carries out a cost-benefit analysis of building a new high-speed railway network. The analysis gives the following details. If the railway network is built, the government insists that the net social benefit exceeds US$20 billion. What level must the external costs reach in order to meet the government's requirement?
- Aunder US$40 billion
- Bover US$40 billion but below US$60 billion
- Cover US$60 billion but below US$160 billion
- Dover US$160 billion