资料题：According to the passage, children must be taken to school by car when（　）。

The United States is full of cars.There are still many families without cars, but some families have two or even more.However, cars are used for more than pleasure.They are a necessary part of life.Cars are used for business.They are driven to offices and factories by workers who have noother way to get to their jobs.When salesmen are sent to different parts of the city, they have to drive in order to carry their products.Farmers have to drive into the city in order to get supplies.Sometimes small children must be driven to school.In some cities, school buses are provided only when children live more than a mile from the school.When the children are too young to walk that far, their mothers take tums driving them to school.One mother drives on Monday, taking her own children and the neighbors' children as well.Another mother drives on Tuesday, another on Wednesday, and so on.This is called forming a car pool.Men also form car pools,with three or four men taking tums driving to the place where they all work.More car pools should be formed in order to put fewer cars on the road and to use less gasoline.Parking is a great problem, and so is the traffic in the cities.Too many cars are being driven.Something will have to be done about the use of cars.