Alia and her 5 friends went for a trip and stayed in a hotel. There were 4 vacant rooms A, B, C and D. Out of these 4 vacant rooms A and B were double share rooms and two rooms C & D can contain one person each. Answer the following questions:
(a) Find the number of ways in which room A can be filled.
(b) It room A and B are already filled each, then find the number of room C can be filled.