Final answer:
The constraint x1 - x2 = 0 implies that the values for x1 and x2 must be equal, which means that if project 1 is selected, project 2 must also be selected, making the statement false.
Step-by-step explanation:
The statement x1 − x2 = 0 implies that the values for x1 and x2 must be the same. If these variables represent the selection of projects 1 and 2, respectively, then this constraint means that if project 1 (x1) is selected, project 2 (x2) must also be selected to satisfy the equation, as they have to be equal. Therefore, the statement that 'the constraint x1 − x2 = 0 implies that if project 1 is selected, project 2 cannot be' is False because they must be selected together to satisfy the equation.