Replace the expressions with the correct modal verb (couldn't, can, need, needn't, ought to, must)

1. It is necessary for you to have a computer password.
   You [ ] to have a computer password.
2. It isn't necessary to stay in the exam hall after you have completed the test.
   You [ ] stay in the exam hall after you have completed the test.
3. I advise you to seek help if you are having problems understanding something.
   You [ ] seek help if you are having problems understanding something.
4. Nia is a very well-behaved child. I doubt that she was nasty to the other children.
   Nia is a very well-behaved child. She [ ] have been nasty to the other children.
5. Richard has the ability to solve maths problems very quickly.
   Richard [ ] solve maths problems very quickly.
6. Students are obliged to be on time for class.
   Students [ ] be on time for class.