Explanation:

- "To keep the ball rolling" means to continue an activity or keep things progressing.

- Option 1: To earn more and more

? This isn't the correct meaning. "Keep the ball rolling" isn't about earning money.

- Option 2: To work constantly

? This is related but not specific enough. It's not just about working, but about continuation.

- Option 3: To keep the conversation going

This correctly reflects the idiom. It means to maintain progress, often in a discussion or activity.

- Option 4: To make the best use of

? This involves efficiency or advantage, not just continuation.

Correct answer: Option 3: To keep the conversation going