Here are the correct answers for each sentence:

31. If we don't book our tickets in advance, we **might not** guarantee seats for the concert. (D)
32. If you want to have a productive meeting, you **must** carefully make a plan and set clear goals. (C)
33. If the traffic is heavy, we **might** make it to the movie theater before the show starts. (B)
34. If you want to sleep well at night, you **shouldn't** drink too much caffeine after 3 p.m. (A)
35. If you don't have a membership, you **can't** access this exclusive club. (C)
36. If they want to participate in the competition, they **mustn't** violate any of these regulations. (B)
37. If you want to avoid traffic, you **should** leave early in the morning or consider another route. (C)
38. If you want to succeed in their career, you **should** consider pursuing further education. (D)
39. If the team works effectively, they **can** complete the project ahead of schedule. (C)

Let me know if you need further help!