SAT Text Structure and Purpose Question

In Egypt, use of solid fuel (e.g., coal, wood) as a share of total household fuel use fell by more than 90 percent between 2000 and 2015; such shifts are typically explained by appeal to the energy ladder, a model holding that fuel choice is mediated mainly by household income (specifically, high-technology fuels displace solid fuels as incomes rise). Alemu Mekonnen and Gunnar Köhlin's study of fuel use in Ethiopia shows how reductive this model is, however: although income of course constrained fuel choice, several circumstances, including the local availability of different fuel sources, influenced decisions. Which choice best describes the function of the underlined portion in the text as a whole?