Here are the correct options for each question:

21. A. happier
22. C. more carefully
23. A. didn't see
24. C. concentrate
25. B. picking
26. A. a
27. B. picking
28. B. not
29. A. spend
30. D. to go
31. C. more well
32. B. truly
33. (No answer since it’s a writing prompt)
34. A. watch
35. (The sentence is already written correctly.)
36. (The sentence is already written correctly.)
37. (The sentence is correctly transformed.)
38. (The sentence is already written correctly, just needs to maintain the original meaning.)
39. (The sentence needs minor adjustments but generally expresses the intended meaning.)
40. (The sentence expresses the intended meaning clearly.)

If you need more detailed explanations for each answer, let me know!