Ответ:
a).
1. What were you doing at 5 yesterday?
2. What were they doing when you came in?
3. What were you doing when I rang you up?
4. What were you doing at that time?
b).
1. When will you be taking your friends out to dinner?
2. What will you be doing if they come at nine?
3. Will they be discussing Comrade Petrov's talk at this time tomorrow?
4. When will you be seeing the engineers?