我们教授留了一条题目做report In what circumstances didMarshall and his followers advocate subsidies for decreasing-cost industries?How did they reconcile this form of intervention with their support for thecompetitive market?

我想从decreasing cost的定义开始分析，然后做出Marginalism的demand and supply的图像，从welfare的角度说明为什么要补贴。然后再分析为什么只有constan cost industries才能维持住competitive market。但是觉得很杂乱无章还有就是我看了马歇尔的《经济学原理》这本书，他主要讨论了土地，劳动力和资本对于output的影响，是不是还有什么地方是可以更加关注一点的？