Take point B as the research object, analyze the stress situation: the tension T of the heavy object (equal to the gravity G of the heavy object), the supporting force N of the light rod and the tension F of the rope, and make a chart as follows:

According to the equilibrium condition, the resultant force of n and f is equal to t, and the directions are opposite. According to the similarity of triangles, Nab = FBO = Tao can be obtained.

T=G, the solution is: N=ABAO? G, F = boao? g；

When ∠BAO decreases slowly, AB and AO remain unchanged, while BO decreases, N remains unchanged and F decreases. So b is right and ACD is wrong.

So choose B.