假设未来两年美国政府多借5000亿美元，并将这笔钱用于基础设施建设 — — 即使这些投资并非生产性的，其社会回报率只有每年25 % 。
Suppose that the US government were to borrow an extra $500 billion over the next two years and spend it on infrastructure – even unproductively, on projects for which the social rate of return is a measly 25% per year.