The speed of boat a down the stream is 125% of the speed in still water. If the boat takes 30 minutes to cover 20 km in still water, then how much time (in hours) will it take to cover 15 km upstream?

A

$$\frac{3}{4}$$

B

$$\frac{1}{2}$$

C

$$\frac{1}{4}$$

D

1