1. **Problem Statement:** We have a constraint in a 0-1 integer programming model: $$x_1 - x_2 \leq 0$$ where $x_1$ and $x_2$ are binary variables representing whether project 1 and project 2 are selected (1) or not (0). 2. **Understanding the Constraint:** The inequality means the value of $x_1$ minus the value of $x_2$ must be less than or equal to zero. 3. **Rewrite the Constraint:** $$x_1 - x_2 \leq 0 \implies x_1 \leq x_2$$ 4. **Interpretation:** Since $x_1$ and $x_2$ can only be 0 or 1: - If $x_2 = 1$ (project 2 is selected), then $x_1$ must be less than or equal to 1, so $x_1$ can be 0 or 1. - But if $x_2 = 1$ and $x_1 = 0$, then $x_1 \leq x_2$ holds (0 \leq 1). 5. **Check all cases:** - If $x_2 = 1$ and $x_1 = 0$, constraint holds. - If $x_2 = 1$ and $x_1 = 1$, constraint holds. - If $x_2 = 0$ and $x_1 = 1$, constraint fails (1 \leq 0 is false). - If $x_2 = 0$ and $x_1 = 0$, constraint holds. 6. **Conclusion:** The constraint forbids $x_1 = 1$ when $x_2 = 0$. In other words, if project 1 is selected, project 2 must be selected. But if project 2 is selected, project 1 may or may not be selected. 7. **Answer to the question:** "If project 2 is selected, project 1 must be selected" is **false**. "If project 2 is selected, project 1 must not be selected" is **false**. "None of the above" is the correct choice. **Final answer:** None of the above