Final answer:
The restriction X₁₁, X₁₂, X₂₁, X₂₂≥0 in the model means that each decision variable must be nonnegative in the optimal solution.
Step-by-step explanation:
The restriction X₁₁, X₁₂, X₂₁, X₂₂≥0 in the model means that each decision variable must be nonnegative in the optimal solution. This means that the values of X₁₁, X₁₂, X₂₁, and X₂₂ can be zero or positive.