use the 1st derivative test. the interval on the left of the critical point should be positive when i substitute a value less than my critical point into the revenue, indicating an increase in the marginal revenue. the interval on the right of the critical point should be negative when i substitute a value greater than my critical point into the revenue, indicating a decrease in the marginal revenue. b. use the 1st derivative test. the interval on the left of the critical point should be negative when i substitute a value less than my critical point into the marginal revenue, indicating an increase in the revenue. the interval on the right of the critical point should be positive when i substitute a value greater than my critical point into the marginal revenue, indicating a decrease in the revenue. c. use the 1st derivative test. the interval on the left of the critical point should be negative when i substitute a value less than my critical point into the revenue, indicating an increase in the marginal revenue. the interval on the right of the critical point should be positive when i substitute a value greater than my critical point into the revenue, indicating a decrease in the marginal revenue. d. use the 1st derivative test. the interval on the left of the critical point should be positive when i substitute a value less than my critical point into the marginal revenue, indicating an increase in the revenue. the interval on the right of the critical point should be negative when i substitute a value greater than my critical point into the marginal revenue, indicating a decrease in the revenue. e. i can check to see if the second derivative is positive,