Все предложения соответствуют одному правилу: продолжительное действие в прошлом (PC), которое было прервано «коротким» действием (PS)

1)came , were writing

2)returned, were doing

3)were buying, saw

4)rang, was taking

5)was watering, knocked

1 came

were writing

2 returned

were doing

3 were buying

saw

4 rang

was taking

5 was watering

knocked