And here's an interesting ratio: Starting from zero in 1900, only 30 years later, the ratio of motor vehicles to the number of households in the United States reached 90 percent in just 30 years.
这是个有趣的比例：1900年，没有一个家庭拥有汽车； 仅仅三十年之后， 90%的美国家庭都拥有了汽车。