题目内容：
Before the 20th century the horse provided day to day transportation in the United States.Trains were used only for long-distance transportation. Today the car is the most popular 21 of transportation in all of the United States. It has completely 22 the horse as a means of everyday transportation. Americans use their car for 23 90 percent of all personal 24 .
Most Americans are able to 25 cars. The average price of a 26 made car was1,050 in 1950, 1,740 in 1960 and up to 1,750 27 1975. During this period American car manufacturers set about 28 their products and work efficiency.
As a result, the yearly income of the 29 family increased from 1950 to 1975 30 .than the price of cars. For this reason 31 a new car takes a smaller 32 of a family's to- tal earnings today.
In 1951 33 it took 8.1 months of an average family's 34 to buy a new car. In1962 a new car 35 8.3 of a family's annual earnings. By 1975 it only took 4.75 months in- come. In addition, the 1975 cars were technically superior to models from previous years.
The influence of the automobile extends throughout the economy as the car is so important to Americans. Americans spend more money to keep their cars running than on any other item.
根据以上内容,回答题.
第21题应填入( ) A．kinds
B．means
C．mean
D．types
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答案解析：