Is this a math question? lol i bet you'd find a better answer if you put it in the math section. Good luck :)
I don't know much about the Sydney mines, but from the context, I can infer that the tunnels were unsafe. Whether being that they were collapsing, or they were filled with poisonous gasses.
Answer:
Possibly because if something stained the wall, then it would not come off, would be hard to get off, or will not come off for a long time.
C. "While you finish your chores," said my older brother, "I will help fix your video game."
There there needs to be the comma inside of the quotation marks at the end of the sentence, yet the comma outside of the quotation marks when it is at the beginning of the phrase. There should also be quotation marks in front of the next quotation.