Which of the following is a strong indicator that a building is unsafe to enter?

A strong indicator that a building is unsafe to enter is the presence of a placard indicating explosive material. This signifies that the area poses an immediate threat due to hazardous materials that can cause explosions or similar dangers. Such signage serves as a warning to individuals approaching the building, indicating that there may be volatile substances on the premises that require special handling and precautions.

In contrast, while a closed door might suggest that a building isn't currently occupied or that entry could be restricted, it doesn't inherently indicate danger. Wind direction is important for assessing smoke movement or fire behavior but doesn't directly reflect the safety of entering a structure. Similarly, the color of smoke can provide useful information about what is burning but is not definitive enough to serve as a singular indicator of whether it is safe to enter a building.