Let x1 be the number of units to make and x2 be the number of units to buy. If it costs $1.5 to make a unit and $4 to buy a unit and 4,000 units are needed, what is the objective function?
a. Max 6000x1 + 16000x2
b. Max 1.5x1 + 4x2
c. Min 4000 (x1 + x2)
d. Min 6000x1 + 16000x2
e. Min 1.5x1 + 4x2