Refer to the Following Paragraph to Answer the Questions Below

Refer to the following paragraph to answer the questions below.
Mary Rodale studied discrimination against women plumbers. She hypothesized that individuals who believe strongly in traditional sex roles would be more discriminatory than individuals who showed strong non-traditional sex roles. She contacted 100 individual home owners who planned to hire a plumber. She offered each subject six possible plumbers to choose from. The plumbers had varying levels of experience and training and charged different rates. One of the six was a woman and the other five were men. Every single home owner who believed strongly in traditional sex roles chose a male. All those who believed in non-traditional sex roles ignored gender and chose on the basis of skill and price alone.
-What is the independent variable in the study?

A) the amount charged by the plumber
B) whether a small business owner was male or female
C) whether or not discrimination against women plumbers occurred
D) traditional versus nontraditional sex roles