Sachin takes 50 minutes more than Prem to walk 35 km. But if Sachin doubles his speed, he is ahead of Prem by $$1\frac{1}{2}$$ hours. Sachin's speed is:

A

7 km/h

B

$$7\frac{1}{2}$$ km/h

C

$$8\frac{1}{2}$$ km/h

D

8 km/h