The success of the automobile industry in the 1920s was due to

Mass production and easy credit

Cheap gasoline and low government regulation

Monopolistic practices

Sophisticated advertisement campaigns

Mass production and easy credit.

Explanation:

Before the 1920s, cars were very expensive. In the 1920s, the industry turned to mass production because it was really more profitable to produce millions of financially affordable cars than producing a few thousands for the rich people. Technology advances such as the assembly line and design innovations, plus sales strategy and easy credit, made this possible. In 1918, only 1 in 13 families owned a car. In 1929, 4 out of 5 families had a car.