Which of the following is not acceptable as a constraint in a linear programming problem:

Which of the following is not acceptable as a constraint in a linear programming

    problem:

      Constraint "1"      X1 + X1X2 + X2 => 12

      Constraint "2"      X1 - 2X2 =< 20

      Constraint "3"       X1 + 3X2 = 48

      Constraint "4"       X1 + X2 + X3 =< 150

      a. Constraint "1"

      b. Constraint "2"

      c. Constraint "3"

      d. Constraint "4"