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 28898 (d) What is happening to the average rate of change of sales as the number of catalogs distributed increases? 53 56 Sales (in thousands) 30- 35 20- (a) The average rate of change over 5 to 15 is (Type an integer or a simplified fraction.)