Which situation is the clearest example of cause and effect in "the city without us"?

Am assuming you mean 'the world without us'. Assuming this, the book emphasizes an idea called 'deconstruction' This is the best example of simple cause and effect where entire urban areas over time that are completely abandoned, i. e., "without us" or without humans. With deconstruction over time water would leak into a building, say a home, eventually rotting away the wood and less dense materials leaving only metals like aluminum and copper. Another effect of the deconstruction is that without humans to provide food and warm creatures like mice and cockroaches would die off.