A、Pop sound. B、Broken finger. C、Pulling-apart of bones. D、Cracked bone. C由女士所说的“When the bones of the joint are pulled apart，and

W: Don’ t crack your knuckles! You’ ll end up with arthritis when you’ re older. M. I don’ t know why I do it. It just feels nice.
W: Now we can use MRI imaging to see what is actually happening inside the joint when someone cracks their knuckles.
M: Then what is happening?
W: Well, when you crack your knuckles, it stretches the space around the joint and surrounding fluid and causes the formation of bubbles. Now when you stretch the joint far enough, these bubbles burst, producing the "pop" sound.
M: Is there any damage to the joint?
W: When the bones of the joint are pulled a-part, and there’ ll be a little bit of fluid in between, which can damage the surfaces of the joints.
Q13: What is the conversation mainly about?
Q14: What happens when you stretch the joint far enough?
Q15: What could cause the damage to the surfaces of the joints?