(1 point) A company estimates that it will sell N(x) units of a product after spending x thousand dollars on advertising, as given by N(x)=-5x+215x² - 3900x + 16000, (A) Use interval notation to indicate when the rate of change of sales N'(x) is increasing. Note: When using interval notation in WeBWorK, remember that: You use 'I' for oo and '-' for -co, and 'U' for the union symbol. If you have extra boxes, fill each in with an 'x'. N'(x) increasing: 10 ≤ x ≤40. (B) Use interval notation to indicate when the rate of change of sales N'(x) is decreasing. N'(x) decreasing: (C) Find the average of the x values of all inflection points of N(x). Note: If there are no inflection points, enter -1000. Average of inflection points= (D) Find the maximum rate of change of sales. Maximum rate of change of sales-