What Influence Do Pairs of Shepherd Moons Have on the Giant

Multiple Choice

What influence do pairs of shepherd moons have on the giant planets' rings?

A) They gravitationally pull material off each other, reinforcing the rings.
B) They only allow the rocky ring systems to remain stable while destabilizing the icy ring systems.
C) They cause the rings to eventually fall into Saturn by gravitational tugs on the ring particles.
D) They keep rings between the pair narrow by gravitational tugs on the ring particles.
E) They prevent rings from forming between their respective orbits.