The following questions refer to a capital budgeting problem with six projects represented by binary variables a, b, c, d, e, and f. a. Write a constraint modeling a situation in which two of the projects 1, 3, 5, and 6 must be undertaken. b. Write a constraint modeling a situation in which, if project 3 or 5 is undertaken, they must both undertaken. c. Write a constraint modeling a situation in which project 1 or 4 must be undertaken, but not both. d. Write constraints modeling a situation where project 4 cannot be undertaken unless projects 1 and 3 are also undertaken. e. Revise the requirement in part (d) to accommodate the case in which, when projects 1 and 3 are undertaken, project 4 must also be undertaken