Choose the most appropriate answer:A steamer moves with a speed of 4.5 km/h in still water to a certain upstream point and comes back to the starting point in a river which flows at 1.5 km/h. The average speed of steamer for the total journey is

• A. 12 km/h
• B. 9 km/h
• C. 6 km/h
• D. 4 km/h