WHAT DO YOU KNOW?

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Across
  1. 5. 31PP
  2. 6. 31PA
  3. 7. 3PRB
  4. 9. 3NIC
Down
  1. 1. 31CL
  2. 2. 3BKR
  3. 3. 31EP
  4. 4. 3ISD
  5. 8. 3ATY