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) 15 to 25 (b) 25 to 35 (c) 35 to 45 (d) What is happening to the average rate of change of sales as the number of catalogs distributed increases? (a) The average rate of change over 15 to 25 is (Type an integer or a simplified fraction.) 70 60- 50- 40- 30- (spuesnou u) sayes 20- A25 10+ 0- 138 143 47 0 10 20 30 40 50 60 Number (in thousands).