Which summary of the 19th century most clearly emphasizes the theme of economic change

Incomes of u.s. households in the 1970s and 1980s a. grew rapidly, due to the widespread success of labor unions in pushing up wages during those decades. b. grew rapidly, due to several increases in the minimum wage during those decades. c. grew rapidly, due to government policies that discourage the importation of foreign products during those decades. d. grew slowly, due to slow growth of the output of goods and services per hour of u.s. workers' time during those decades.