Sudhin cycled at 8.5 km/h for 4 hours, then took an auto and travelled at 20 km/h for 1.5 hours, and then walked at 4 km/h for y hours. If Sudhin's overall average speed for the entire journey was 10 km/h, what is the value of y?

A

2

B

1.25

C

1.5

D

1.15