Here are the mistakes for each sentence:

31. **D** ("play" should be "plays")
32. **C** ("three months ago" should be "three months ago.")
33. **A** ("interest" should be "interesting")
34. **A** ("sister" should be "my sister")

So the answers are: 31 - D, 32 - C, 33 - A, 34 - A.