There are two time-series, X and Y. You are interested in testing the null hypothesis that the average of the difference between the two variables is equal to 3. What should you do?

a. Regress the time-series of the difference between Y and X on a constant

b. Regress the time-series of Y on a constant

c. Regress the time-series of Y on the time-series of X

d. Regress the time-series of X on the time-series of Y