AB。 According to the stress analysis of heavy objects, the resultant force of the left and right ropes on the heavy objects is equal to the gravity of the objects in opposite directions, so the resultant force of the light ropes on the objects remains unchanged, so A is wrong and B is wrong;

CD, taking heavy objects and ropes as the research objects, analyzes the stress situation: gravity, the force FA of cylinder A on the rope and the force FB of cylinder B on the rope. According to symmetry, FA=FB.

According to the equilibrium condition, 2FAcosα=G, α is the angle between the force of the cylinder on the rope and the vertical direction, and g is the gravity of the object. The longer the rope, the smaller the α, the larger the cosα and the smaller the FA.

So the longer the rope, the smaller the tension of the light rope, so C is wrong and D is right.

Therefore, choose: d.