A student walked 6 km from his house to reach the metro station, then boarded a metro that has an average speed of 60km/h, and reached the destination. It took 3 hours for the entire journey. If the average speed of the entire journey is 32 km/h, then the speed of walking is ________.

A

1.5 km/h

B

3 km/h

C

2 km/h

D

4 km/h