A jet boat is used to take canoers 200 miles upriver to their starting point. As the driver of the jet boat,
 Jeremy knows that it will take him 5 hours to get the canoers upstream, but only 4 hours to get the boat
 back downstream after dropping the canoers off, as long as he keeps the boat moving at a constant speed.
 How fast would the boat be moving in still water?
 A
 B
 C
 D
 5 mph
 40 mph
 45 mph
 50 mph