Поставьте глаголы в Past Indefinate,Past Continuous или Past Perfect when does coach (to return) the students (to do)

1. When the coach returned the students were doing the forth exercise. 2. The students had done all the exercises before the coach returned. 3. We discussed two important questions at last meeting. 4. The runner had already set a new record before his friend came to the contest. 5. We had discussed all the problems by 5 o\'clock in the evening. 6. When you phoned the students were doing morning exercises.