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. Which of the following pair of constraints enforces the quantity discount relationship on X3?
a. X31 <= M3Y3 , X32 >= 50Y3
b. X31 <= M3Y3 , X31 >= 50
c. X32 >= (1/50)X31 , X31 <= 50
d. X32 <= M3Y3 , X31 >= 50Y3