找到一个关于price ceiling的case来做分析，为什么市场需要有价格上限，政府介入的结果是什么。

再找一个关于positive externality的case分析，必须是中国的，讨论政府对存在的正外部效应的调整。

具体问题如下，每一个part要求1000字：

PART A
Explain why governments sometimes impose a price ceiling in a competitive market. Illustrate the
effects with the diagram.
Choose a case study where a price ceiling has been used. Identify why the price ceiling was thought to
be necessary in this market. Identify the results of the government intervention. Suggest a better way
of dealing with the issue in your case study.

PART B
Explain why government intervention is often required if there is a positive externality in the market,
and illustrate with a diagram. Briefly list what are the ways to correct for a positive externality and
discuss the options.
Choose a case study from your home country where a positive externality occurs in a market. Discuss
ways your government has addressed the presence of positive externalities. Suggest a better way of
dealing with the issue from your case study.