20th century本题问的是在美国家庭汽车是什么时候普及的，属于细节题。根据文中"The family car has been the common thing since the early 20th century．”可得知答案。

Mericans depend on their cars more than other people. The family car has been the common thing since the early 20th century, and it has changed file American life. Many people have move outside the large cities to the suburbs. Some Americans spend two hours a day or more in their cars, going to work and back home. Cars have become the means of transportation for most Americans when they go shopping and even go on vacations. In the past, Americans liked the big ears, and the gas wasn’t expensive. However, the price of gas has greatly increased in recent years. Smaller and foreign cars have become very common. That is why Japanese and German cars sell well in the U.S.