In a binary integer programming problem, which of the following constraints would enforce a mutually exclusive (one or the other) relationship between project 1 and project 2? Select one: a. x1 − x2 = 1 b. None of the answer choices is correct. c. x1 x2 ≥ 1 d. x1 − x2 ≤ 1 e. x1 x2 ≤ 1