adjective or adverb?

Put the correct word in each gap.
  1. Liz even knows how to answer the most (difficult/difficultly) questions.
  2. She is a (friendly/in a friendly way) and (efficient/efficiently) secretary.
  3. They walked (happy/happily) across the fields.
  4. After some time the girl's face grew (sad/sadly).
  5. Nelly prepared the salad very (good/well).
  6. Jill is a (beautiful/beautifully) girl.
  7. Have you ever seen such (beautiful/beautifully) flowers?
  8. Frank bought the house and renovated it (nice/nicely).
  9. He didn't do his homework (correct/correctly).
  10. She played the piano very (good/well).