Why are floods in urban areas particularly dangerous?

A. city buildings do not allow flood waters to recede B. pollutants and diseases are spread by flood waters more rapidly C. crop soils are permanently damaged D. most cities are built in flood zones

In urban areas, there are more people, therefore not a lot of farmland. (that eliminates C) If city buildings don't allow flood waters to recede, that could be dangerous in many ways. (So A is still a reasonable answer) If most cities are built in flood zones, there should be some form of shelter if houses aren't safe. Pollutants and diseases being carried in water are already dangerous, but easier to spread with rapid flood waters that are sweeping everywhere. So your best answer would be either A or B.