B, the long board is also in equilibrium at this time, that is, the long board by the tension and the two friction forces are balanced, so at this time, the long board by the ground on the ground on its sliding friction is equal to the F-FT, so the ground by the size of the sliding friction force is F -FT, so B is correct;
C, D, sliding friction is proportional to the positive pressure, with the size of the speed and the size of the horizontal tension are not relevant, so C is wrong, D is correct;
The choice of BD.
The sliding friction is equal to F-FT, so the size of the sliding friction on the ground is F-FT.