完形填空
The United States is full of automobiles (机动车). There are still many families without cars. But some families have two or1more cars. However, cars are used for more than pleasure. They are a2part of life.
Cars are used for3. They are driven to offices and factories by workers who have no other way to4their 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 shop for necessities.
Sometimes, small children must be driven to5. In some cities, school buses are used only when children6__more than a mile from the school. One7drives on Mondays, taking her children and the neighbors' children as well. Another mother drives on Tuesdays, another on Wednesdays and so on. This is called forming a car pool (拼车). Working people also form car pools, with three or four people taking turns driving to the place8they work.
More car pools should be formed in order to put9cars on the road and to use less oil.
10 is a great problem, and so is the traffic in and around cities. Too many cars are being driven. Something should be done about the use of cars.
为植物必需的矿质元素的一般实验方法是（ ）