Which of the following are examples of acceptable OLS regression models? a) ???? = ????1 + ????2???? + ????3???? + ???? b) ???? = ????1????2 + ????3???? + ???? c) ???? = ????1 + ????2???? + ????3????2 + ???? d) ???? = ????1 + ????2 ln(????) + ????3 ln(????2) + ????