Model verb exercise

<http://www.englishpage.com/modals/interactivemodal8.htm>
My score is 76%. I got 19 out of 25. 6 wrong sentenses left:

2. The book is optional. My professor said we could read it if we needed extra credit. But we [cannot / must not / don't have to] read it if we don't want to.
3. Susan [couldn't / can't / might not] hear the speaker because the crowd was cheering so loudly.
8. You [can't / couldn't / won't be able to] do the job if you didn't speak Japanese fluently.
11. You [may / would / might] eave the table once you have finished your meal and politely excused yourself.
18. I can't stand these people - I [have got to / might / had better] get out of here. I'm going to take off for awhile while you get rid of them.
21. Terry and Frank said they would come over right after work, so they [can / have to / should] be here by 6:00.

Here are my right sentenses:

1. Ted's flight from Amsterdam took more than 11 hours. He must be exhausted after such a long flight.
4. The television isn't working. It must have been damaged during the move.
5. Kate: Can you hold your breath for more than a minute?Jack: No, I can't.
6. You don't have to be rich to be a success. Some of the most successful people I know haven't got a penny to their name.
7. I've redone this math problem at least twenty times, but my answer is wrong according to the answer key. The answer in the book must be wrong!
9. You shouldn't worry so much. It doesn't do you any good. Either you get the job, or you don't. If you don't, just apply for another one. Eventually, you will find work.
10. You have to be kidding! That can't be true.
12. Jenny's engagement ring is enormous! It must have cost a fortune.
13. Shall we move into the living room? It's more comfortable in there and there's a beautiful view of the lake.
14. If I had gone white water rafting with my friends, I would be floating down the Colorado River right now.
15. At first, my boss didn't want to hire Sam. But, because I had previously worked with Sam, I told my boss that he ought to take another look at his resume and reconsider him for the position.
16. You had better take along some cash. The restaurant may not accept credit cards.
17. The machine can be turned on by flipping this switch.
19. You mustn't forget to pay the rent tomorrow. The landlord is very strict about paying on time.
20. Do you always have to say the first thing that pops into your head. Can't you think once in a while before you speak?
22. Yesterday, I had to cram all day for my French final. I didn't get to sleep until after midnight.
23. We shall no longer suffer the injustice of oppression! Freedom shall be ours!
24. If I had gone to the University of Miami, I could have participated in their Spanish immersion program.
25. The lamp might not be broken. Maybe the light bulb just burned out.