Which of the following is a sign of a fly infestation?

The presence of maggots and swarms of flies is a strong indication of a fly infestation. Flies are known for breeding rapidly, and their larvae, or maggots, can often be found in areas where they have access to food sources, especially decaying organic material or waste. The sight of swarming flies is an obvious visual cue, signaling that there is a breeding ground nearby, as flies are naturally attracted to certain smells and sights associated with decay. This combination of adult flies and their larvae not only confirms an infestation but also highlights the need for immediate action to prevent further contamination and maintain food safety.

Other indicators, like droppings or nesting materials, might suggest the presence of different pests or simply be remnants of their activity rather than direct evidence of an active infestation. The distinctive sight and behavior of flies alongside their larvae make option B the most definitive sign of a fly infestation.