The total mass of a skier is 60.0 kg. He starts from rest and slides down a slope without exerting any effort. After he has descended through a total vertical displacement of 50.0 m, he is travelling at a velocity of 30.0 m/s.
How much work has been done against resistive forces experienced by the skier as his velocity increases from 0 to 30.0 m/s?
[Take the gravitational field strength g as 10 N/kg.]

A 3000J
B 30000J
C 27000J
D 57000J
E 2700J

I got the final answer 54000J from w=fxd = 600x90, but it says A. Why?

Well im assuming that the G Potential Energy at the top is converted to the Kinetic Energy down the slope - in an ideal system this conversion is 100%, all the GPE converted to KE , but in this case as theres a difference between these two figures, im taking that as the work done/energy consumed by resistive forces.