1. A man walks on a straight road from his home to market 2.5 km away with a speed of 5km/h. Finding the market closed, he instantly turns and walks back home with a speed of 7.5 km/h. The average speed of the man over the interval of time 0 to 40 min is equal to :
A) 5 km/h
B) 25/4 km/h
C) 30/4 km/h
D) 45/8 km/h