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