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)=−3x^3+210x^2−4000x+19000,10≤x≤ 40. (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 [infinity] and '-I' for −[infinity], and 'U' for the union symbol. If you have extra boxes, fill each in with an 'x'. N′(x) increasing:. (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 =.