Which of the Following Factors Was Essential to the Emergence

Which of the following factors was essential to the emergence of a "white-collar" service sector in the late 1800s?

A) The decline in manufacturing, due to the economic recession of the early 1870s
B) Compulsory primary school education, which allowed workers to acquire literacy and mathematical skills they could use in offices
C) The growing importance of labor unions and the rebellion of factory workers against the harsh working conditions imposed on them
D) The emergence of the consumption economy, and especially the department store, as salesgirls were needed to attract shoppers