问答题 人口学家预言人口中，老年人的比例在未来20年中将要提高。生命周期模型预测这种人口变化对国民储蓄率会有什么影响?
   Demographers predict that the fraction of the population that is elderly will increase over the next 20 years. What does the life-cycle model predict for the influence of this demographic change on the national saving rate?