Consider the following equation: S; = Bo + BLE + B, A + B3P, +4 where: Si is the annual pre-tax salary earned by individuali E, is the number of years of education completed by individual i A, is individual i's age, in years P, is a dichotomous variable coded 1 if the individual works in the private sector, o if public sector ; is an error term The researcher wants to test the hypothesis that people who work in the public sector earn less than those who work in the private sector, even when accounting for education and age. Which of the following is the main explanatory variable in the researcher's model? SE OB OE PA