A company will be able to obtain a quantity discount on component parts for its three products, X1, X2 and X3 if it produces beyond certain limits. To get the X1 discount it must produce more than 50 X1's. It must produce more than 60 X2's for the X2 discount and 70 X3's for the X3 discount. How many decision variables are required in the formulation of this problem?

a. 3 b. 6 c. 9 d. 12

ANSWER: C

Why the answer is c here, can you detail what are these 9 decision variables. Is the decision variable different with the binary variable, what difference are they.