Are they cooks?

Question

1. No, they are not.
2. No, they're not.
3. No, they aren't.
[Are the three answers grammatical?]

4. Max isn't my pet.
5. Max's not my pet.
[Is #5 grammatical?]

6. Betty and I aren't late for school.
7. Betty and I're not late for school.
[Is #7 grammatical?]

Answer 1

Let's see your answers first.

Answer 2

1-6 are fine.

7 is not.

Answer 3

Thank you.

4. Max isn't my pet.
5. Max's not my pet.
[Is #5 grammatical?]
5-1. Max' not my pet. [What about this one?]
How do you pronounce "Max's"?