challenge: What time period has been studied using speleothems in Crevice Cave to determine climate changes?
  
  A. 100,000 to 150,000 years ago
  B. 25,000 to 75,000 years ago
  C. 50,000 to 100,000 years ago
  D. 10,000 to 20,000 years ago
  Answer: 