小弟刚刚接触公共经济学，目前遇到一个很简单的题，但不知道如何准确的回答，请各位大牛们帮帮小弟：

为什么说在一个经济体里，航空公司在同一架航班上卖不同价格的票给不同的消费者， 会造成没有交换最优？

另外还有一个： For each of the government programs listed below, identify the economic justification for government intervention. This will always include at least one market failure (to achieve Pareto efficiency) and may include reasons to reject a particular Pareto Efficiency result (equity and consumer sovereignty). When there is more than one explanation, explain which you believe is the most important.

A, A Requiring automobile seatbelt use
B, Regulations on auto emissions levels
C, National Defense
D,Unemployment compensation
E, Social Security
F,National weather service
G, Post Office

谢谢！！！