Which of the following statements concerning the variables in a regression analysis is true?
a. If the variables are difference-stationary, then we should always estimate a relationship between the first-differenced series.
b. If the variables are I(1) and cointegrated, then we should first de-trend the series and then perform regression analysis on the de-trended variables.
c. If the variables are trend-stationary, then we should estimate a regression model that includes a trend variable.
d. all of the above.
e. none of the above.