回忆例18.5中的全球变暖问题，教材表18-3显示了每年减少1%温室气体排放所带来的净收益，什么样的

Refer back to Example 18. 5 on global warming. Table 18 -3 (page 623 ) shows the annual net benefits from a policy that reduces CHG emissions by I percent per year. At what discount rate is the NPV of this policy just equal to zero？