challenge: What is the primary purpose of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction model?
  
  A. To describe the behavior of conduction electrons in metals
  B. To model the coupling of nuclear magnetic moments or localized inner d- or f-shell electron spins through conduction electrons
  C. To predict the thermodynamic properties of superconductors
  D. To explain the formation of magnetic domains in ferromagnets
  Answer: 