听力原文：The farm economy of the United States has changed a lot in the last seventy years. In the 1930s, 25 percent of the nation's population lived on farms. Today less than one percent of Americans do.
Farm incomes have changed over the years too. For example, in 1933, people living and working on farms had much less money to spend than other Americans. At that time, farm families had about one-third the income of non-farmers after all necessary expenses had been paid. By the late 1970s, however, that difference had almost disappeared.
In 2004, farmers had their best year ever. The United States Department of Agriculture says the average farm family earned about 81, 000 dollars. That is more than the average American family, which earned about 60,000 dollars. Yet these numbers do not completely explain the situation for all farmers. Those who have small farms often take other jobs to earn extra income. And farm earnings for large farms grew faster than for mall ones.
The Department of Labor measures the pay of industrial workers differently. It measures the average hourly and weekly pay for industrial workers. This is because factory workers are generally paid by the hour unlike farmers who earn income from their farm businesses. Industrial workers are about 23 percent of the labor force. But that number has been decreasing. Most Americans have jobs that provide services. Professional, technical and other services employ about 76 percent of the labor force.
How many Americans were living on farms seventy years ago？
(33)

About 25 percent of the population.
B. Less than 23 percent of the population.
C. About 76 percent of the population.
D. Less than 1 percent of the population.