Choose the correct options

1. I can't see the light in the windows. They
   [can have|must have]
   left home.
2. They
   [couldn't have|must have]
   broken up. I saw them together yesterday.
3. She
   [must have|can't have]
   bought such an expensive mobile herself. She is just a pupil.
4. They
   [were able|weren't able]
   to drive there because their car broke down three days ago.
5. He
   [had to|didn't have to]
   come to me. I didn't ask him to do so.
6. I
   [needed to|didn't need to]
   buy milk today. I forgot that I did it the day before yesterday.