(h) The model you fit in part (c) allows us to determine mean salary differences between PhD and Bachelor's managers and between Master's and Bachelor's managers. Which indicator variables would you need to include in a regression model that could be used to determine whether there is a significant difference between the mean salaries of Master's and PhD managers? Which estimated regression coefficient would estimate the difference in mean salaries? You don't actually have to estimate your model for this question. [Hint: Think about how the indicator variable that is left out affects the interpretation of the coefficients for the indicator variables that are included.]