If river is sky, sky is road, road is bottle and bottle is blackboard, then milk can be kept in

In this chain-replacement puzzle, we track: river→sky, sky→road, road→bottle, bottle→blackboard. If milk is kept in a bottle, and bottle becomes blackboard, then milk is kept in blackboard. However, if milk is kept in a river (original), then river→sky→road→bottle→blackboard. The question asks where milk CAN be kept - if we consider the original container (river/bottle), then River (Option A) would be where milk starts before any transformation.