A boy plans to cross a river in a rubber raft. The current flows from north to south at 1 m/s. In what direction should he head to get across the river to the east bank in the least amount of time if he is able to paddle the raft at 1.5 m/s in still water?
(a) Directly to the east
(b) South of east
(c) North of east
(d) The three directions require the same time to cross the river.