A boat sails downstream from a point P to point Q, which is 24 km away from P, and then returns to P. If the actual speed of the boat (in still water) is 6 km/h, the entire trip from P to Q takes 3 hours less than that from Q to P. What should be the actual speed (in km/h) of the boat to sail from P to Q in 2 hours?

A

12

B

15

C

10

D

22