**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”