Look at the question. What's the subject?

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Across
  1. 3. Comment allez-vous?
  2. 6. What is a semi-quaver?
  3. 11. What makes for a happy couple?
  4. 12. How are mountains formed?
  5. 13. Paint a picture of a tree
Down
  1. 1. Shall we play football today?
  2. 2. What is mass?
  3. 4. How do trees use sunlight?
  4. 5. Compare these two novels
  5. 7. Why was Napoleon so popular?
  6. 8. What happens to demand if prices go up?
  7. 9. What is 2+2?
  8. 10. What happens if we burn magnesium in air?