(2003? As shown in the figure, the block of wood with mass m slides along the horizontal ground at a uniform speed under the action of tensile force F, and inclines upward at an angle α with the horizontal direction. Because the object moves in a straight line at a uniform speed, it is in equilibrium and the resultant force is zero.

Horizontal direction: Ff=Fcosα.

Vertical direction: n = mg-fsinα.

According to Newton's third law, the pressure of wood blocks on the ground is n ′ = n = mg-fsinα.

According to Ff=μN, ff = μ (mg-fsinα).

So A, C and D are right and B is wrong.

So choose a, c and d.