When a model has a unique optimal solution, it means that $\_\_\_\_\_\_\_\_.$

Question $20$ options:

A$)$ the Allowable Increase or Allowable Decrease values for changing cells are zero

B$)$ there is no solution that simultaneously satisfies all the constraints

C$)$ the objective is maximized or minimized by more than one combination of decision variables

D$)$ there is exactly one solution that will result in the maximum or minimum objective