Solution: Solution: AB rod is in equilibrium under the action of gravity Mg, rope tension T and the force F exerted by the shaft on the rod, and the resultant force is zero, as shown in the figure. According to the intersection principle of three forces, the angle between the pulling force and the horizontal direction is θ. According to the balance and symmetry, F=T, and in the vertical direction:

2tsin θ = mg。 And T=mg, then f = mg2sin θ.

According to symmetry, F=mg.

According to the equilibrium condition, we know that the resultant force of T and Mg is equal to F. If it is the opposite, we can get from the mathematical knowledge: F=(Mg)2+T2+2Mg? Tcos(90 +θ)=M2g2？ 2Mmgsinθ+m2g2

So choose ABC