"they" dùng was thì magic quó=)

1 Were you having dinner at 7 o'clock yesterday evening?

Yes, I was. / No, I wasn't. I was having dinner at a restaurant.

2 Were you doing homework at 8 o'clock yesterday evening?

Yes, I was. / No, I wasn't. I was playing video games.

3 Were you watching a film at 9 o'clock yesterday evening?

Yes, I was. / No, I wasn't. I was studying for an exam.