The answer is b. because this way you can hear all the characters thoughts and what they are saying. this lets you know how they truly feel versus how they want you to think they feel.
Erosion is when it rains and the rain wears down rocks making it bumpy, smaller, and deformed. With the floor incorporating rock the rain would make the earth deformed causing damage with the ground changing which can cause earthquakes and such. I hoped this helped you understand a little