In a three-storey building, there are four rooms on the ground floor, two on the first and two on the second floor. If the rooms are to be allotted to six persons, one person occupying one room only, the number of ways in which this can be done so that no floor remains empty is a. `"^8P_6-2(6!)` b. `"^8P_6` c. `"^8P_5(6!)` d. none of these