A man starts walking at 3 pm. He walks at a speed of 4 km/hr on level ground and at a speed of 3 km/hr on uphill, 6 km/hr downhill and then 4 km/hr on level ground to reach back to his home at 9 pm. What is the distance covered on one way? Select one: a. 10 km b. 12 km c. 15km