4. The monthly revenue R of a telephone polling service is related to the number of the completed responses by the function R = -23.58 + 60 6x2 + 20x. If the number of completed responses is 30 now and is increasing at the rate of 8 forms per month, what is the rate at which the monthly revenue is changing? $8.66 a) b) $10.82 C) $12.44 9052 d) $5.41