The correct answers are:
A. City buildings do not allow flood waters to recede;
The buildings in the cities are pretty much an obstacles for the flood waters, but they are also making it harder for the waters to recede because of that. This makes a real problem because big portions of the affected city can be under water for much longer period of time, thus more damage to be created, than if it was an open space.
B. Flood waters spread pollutants and diseases more rapidly;
The flood waters can take all sorts of things with them on their way, and very often they can bring in lots of pollutants of different sorts in the cities, and that can make an ecological disaster. Also, because of all the material with it, and creating high humidity, muddy, swampy environment for some time, usually in a period when it is pretty warm, lots of diseases can appear, and if they do, because of the terrible hygienic conditions it can cause quick spreading of them.
