You have to choose the correct Past Continuous Tense or Past Simple Tense form of the verbs.
We ___ \(sit\) around the table and ___ \(have\) dinner when we ___ \(hear\) a knock on the door.

• were sitting/were having/heard
• sat/had/heard
• was satting/had/was hearing
• was sitting/was having/was hearing
• sat/were having/heard