challenge: What is the estimated amount of ore reserves in the Mankhan Uul mine?
  
  A. 500 million tonnes of ore grading 20% P2O5
  B. 100 million tonnes of ore grading 10% P2O5
  C. 2.5 billion tonnes of ore grading 50% P2O5
  D. 1.6 billion tonnes of ore grading 35% P2O5
  Answer: 