Which one of the following statements is not correct?
(a) A linear programming problem with 2 variables and 3 constraints can be solved by Graphical Metho(d)
(b) In big-M method if the artificial variable can not be driven out it depicts an optimal solution.
(c) Dual of a dual is the primal problem.
(d)For mixed constraints either big-M method or two phase method can be employe(d)