Given the Production Function: Q = 21X + 9X2 - X3, where Q = Output, and X = Input . At what value of X does Stage II of the production function begin?
a. Stage II begins at X = 4.5
b. Stage II begins at X = 10.5
c. Stage II begins at X = 21
d. Stage II begins at X = 9
e. none of the above