3. Consider a capital budgeting formulation where the binary variables x1 and x2 are used to represent the acceptance (xi 5 1) or rejection (xi 5 0) of each alternative. The requirement that x2 is contingent upon x1 can be represented by which of the following?

a. x1 1 x2 ,5 1

c. x1 1 x2 .5 1

b. x2 ,5 x1

d. x1 ,5 x2