A firm has prepared the following binary integer program to evaluate a number of potential new capital projects. The firm's goal is to maximize the net present value of its decision while not spending more than its currently available capital. If xi represents the binary decision variable to choose a project, which of the constraints allows only one of the projects represented in the constraint must be chosen?

Max 100x1 + 120x2 + 90x3 + 135x4

s.t.

150x₁ + 200x₂ + 225x₃ + 175x₄ 500 {Constraint 1} x₁ + x₂ + x₃ + x₄ 2 {Constraint 2} x₂ + x₄ 1 {Constraint 3} x₂ + x₃ 1 {Constraint 4} x₁ = x₄ {Constraint 5}