Consider a capital budgeting problem with six project represented by binary (0 or 1) variables X1, X2, X3, X4, X5, X6.
A. Write a constraint modeling the situation in which at least 2 of the project from 1, 2, 4, and 6 must be selected.
B. Write a constraint modeling the situation in which only 2 of the projects from 1, 2, 3, and 4 must be selected.
C. Write a constraint modeling the situation project 2 or 6 must be selected, but not both.
D. Write a constraint modeling the situation in which at most 4 projects from the 6 can be selected.