challenge: What is the primary reason why the American Society of Clinical Oncology discourages the use of CA19-9 as a screening test for cancer?
  
  A. CA19-9 is too expensive to be used as a screening test
  B. CA19-9 is not sensitive enough to detect cancer at an early stage
  C. CA19-9 is not specific to pancreatic cancer
  D. The test may produce false negatives or false positives, reducing its reliability
  Answer: 