The following questions refer to a capital budgeting problem with six projects represented by 0-1 variables x1, x2, x3, x4, x5, and x6. (a) Write a constraint modeling a situation in which exactly three of the projects 1, 3, 5, and 6 must be undertaken. (b) Write a constraint modeling a situation in which, if projects 3 or 5 must be undertaken, then they must be undertaken simultaneously. (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 also are undertaken. (Enter your answer as a comma-separated list of equations and/or inequalities.) (e) In addition to the requirement in part (d), assume that when projects 1 and 3 are undertaken, project 4 also must be undertaken. (Enter your answer as a comma-separated list of equations and/or inequalities.)