Choose the correct options for the sentences so that they have the same meaning and the given ones

1. I'm sure it is very expensive.
   It [may|must|can't]be very expensive.
2. I don't believe it's true.
   It [may|must|can't]be true.
3. I think they are at the mall, but I'm not sure.
   They [may|must|can't]be at the mall.
4. I'm sure they are staying at home.
   They [may|must|can't]be out, they [may|must|can't] be at home
5. I don't know, they could go for a walk.
   They [may|must|can't]be outside for a walk.
6. I'm sure they don't have a dog.
   They [may|must|can't]have a dog.
7. She's not Julia. Julia's blond!
   She [may|must|can't]be Julia. Julia's blond!