How does the word choice in the following passage from "The Open Window" contribute to the mood of the story?

"Their bodies were never recovered. That was the dreadful part of it." Here the child's voice lost its self-possessed note and became falteringly human.

It creates a mood of humor, which is a clue to the reader that Vera is playing a joke on Mr. Nuttel.
It creates a mood of sadness, which allows the reader to believe what Vera is saying.
It creates a mood of tension, which makes the reader suspicious of what Vera is saying.
It creates a mood of mystery, which makes the reader question what Vera is saying.