challenge: What is a necessary condition for a vertex model to be considered integrable?
  
  A. ∀μ, ν, ∃κ such that a specific condition is met
  B. ∀μ, ν, ∃λ such that a specific condition is met
  C. ∃μ, ν, ∀λ such that a specific condition is met
  D. ∀κ, ν, ∃μ such that a specific condition is met
  Answer: 