Physics 'Moments in bodies' A level question

Okay so here goes,
there's a trailer of weight 30 kN that's hitched to a cab at the point of attachment, X. The load on the trailer has a weight of 20 kN. We have to find out the upward force (F) exerted by the cab on the trailer at X.

By taking point A as the pivot and applying the principle of moments, we get:
[(20 x 5) + (30 x 10)] - (F x 20) = 0
100 + 300 - 20F = 0
400 - 20F = 0
F = 20 kN (and that's correct I suppose!)

But the problem is:
I have always thought that it doesn't really matter which axis you choose so I chose point C as the pivot.
[(30 x 0) + (20 x 5)] - (F x 10) = 0
F = 10 kN

So question is, why is the selection of point A as the pivot correct and not that of point C?