challenge: What is the role of Ukrtransgas in Ukraine's gas infrastructure?
  
  A. It is a private company that competes with NJSC Naftogaz of Ukraine in the gas market
  B. It is the sole supplier of natural gas to Ukrainian households
  C. It is an independent Storage System Operator that ensures the operation of Ukrainian underground gas storages
  D. It is a major consumer of natural gas in Ukraine's industrial sector
  Answer: 