Dependent and independent varibles

I have the following problem and I am finding it hard to determine from the question what are the dependent and independent variables in order to put it into a regression equation. This question is:

"A sales representative wonders if he was giving away too many free text books, or not enough. He has determined gross revenue from the sales of the books and the number of free copies for 78 books. He wants to know if there is a direct link between the number of free copies of a book and the gross revenue generated by that book in 2002. To improve the model he has also included the total sales in dollars of 2001 as an additional variable."

Can anyone tell me out of the three variables, REVENUE 2001, REVENUE 2002 and COPIES OF BOOKS which are the dependent and which are the independent variables?