Cars are an important part of life in the

题目：

Cars are an important part of life in the United States. Without a car most people feel that they are poor. And even if a person is poor he doesn’t feel really poor when he has a car.
Henry Ford was the man who first started making cars in large numbers. He probably didn’t know how much the car was going to affect American culture. The car made the United Stated a nation on wheels. And it helped make the United States what it is today.
There are three main reasons why the car becomes so popular in the United States.
First of all, the country is a huge one and Americans like to move around in it. The car provides the most comfortable and cheapest form of transportation. With a car people can go anyplace without spending a lot of money.
The second reason cars are popular is the fact that the United States never really developed an efficient and inexpensive form of public transportation. Long-distance trains have never been as common in the country as they are in other parts of the world. Nowadays there is a good system of air-service provided by planes. But it is too expensive to be used frequently.
The third reason is the most important one, though. The American spirit of independence is what really made cars popular. Americans don’t like to wait for a bus, or a train or even a plane. They don’t like to have to follow an exact schedule. A car gives them the freedom to schedule their own time. And this is the freedom that Americans want most to have.
The gas shortage has caused a big problem for Americans. But the answer will not be a bigger system of public transportation. The real solution will have to be a new kind of car, one that does not use so much gas.

"A nation on wheels" means that ______.

A. the country is producing the best cars in the world
B. everyone in this country owns a car
C. cars play a very important role in people’ lives
D. there are more cars than trains in this country