Silver's Gym Offers Kickboxing I, Kickboxing II, and Kickboxing III $\le$

Silver's Gym offers Kickboxing I, Kickboxing II, and Kickboxing III, and Ms.DeVore insists on teaching all three classes.Otherwise, the gym will offer no kickboxing classes.How would you model this constraint?

A) X₁ + X₂ + X₃ = 0
B) X₁ + X₂ + X₃ = 3
C) X₁ + X₂ + X₃ $\le$ 0
D) X₁ - X₂ = 0 and X₁ - X₃ = 0