It means that it is impossible to get something for nothing, or the idea that even if something seems free, there is always a cost, no matter how indirect or hidden the cost is.
So they won't be seen; so Atticus will be busy reading; because if they were killed they'd miss school instead of vacation and because it is easier to see inside a dark house in the dark rather than in the daytime.
Hope this helps
I believe the genre is Action and Adventure
The sub genre is Westerns
The correct answer is B. "The Wind Gives Life"
That answer fits best the excerpt that you didn't provide, but was quite common among students who asked the same question.
