In April 2016, Mr. Wang noticed that the dead fish often appeared in his fish farm. Environmental protection technicians tested the river water and found the content of toxic chemicals in the river water exceeded the standard, which caused the fish dead. The toxic chemicals came from the waste liquid discharged by a chemical plant located in the upper river. Is this an example of a positive externality or a negative externality? What countermeasures do you have to deal with this externality?

2 . Using the theory of information asymmetry, try to explain the reasons and solutions for adverse selection in the used car trading market?