Read the following passage and answer the item that follows. Your answer to this item should be based on the passage only.

When a city floods after three hours of rain, the rainfall figure is the first number quoted and usually the least useful one. The same quantity of water falling on the same city thirty years earlier drained away, because there were tanks and low lying fields to receive it and channels to carry the rest. Those have since been built over, the remaining drains carry construction debris and silt, and the water that once had somewhere to go now has only the road. Rain is the trigger. What decides whether the trigger produces a flood is what the ground beneath the city has been allowed to become.

Which one of the following statements best reflects the central idea of the above passage?