The figure to the right shows the total sales, in thousands of dollars, from the distribution of x thousand catalogs. Find and interpret the average rate of changes of sales with respect to the number of catalogs distributed for the given changes in x. (a) 5 to 15 (b) 15 to 25 (c) 25 to 35 (d) What is happening to the average rate of change of sales as the number of catalogs distributed increases? Sales (in thousands) 70- 60- 50- 40- 30- 20- 10- 0 25 10 34 20 41 44 30 Number (in thousands) 40 50 60