In City A, the driver's car accident insurance is paid at an average level, rather than charged according to the distance traveled by each driver each year. Under this mechanism, the insurance company can achieve a certain profit. 
to a certain extent, people with driving mileage less than the average level subsidize those who drive more than average.
 For insurance companies, the cost for people who drive less than the average mileage is less than the cost for people who drive more than the average mileage