In the linear regression models we study in this course, y = a + fix, + + fix, + e, which of the following statements conceming the marginal effect of x, on y is most accurate: Select one: O a. It measures the amount of change for y when x, increases by one unit. O b. It measures the amount of change for y when x, decreases by one unit. O c. It measures the amount of change for y when x, decreases by one unit. O d. It measures the amount of change for x, when y increases by one unit. O e. It measures the amount of change for y when r, increases by one unit