Final answer:
Studies indicate firefighters wearing full PPE can have significantly elevated core temperatures after 20 minutes of activity, but the exact temperature isn't provided in the question or references. An increase greater than about 3°C would be excessive and could lead to heat-related illnesses.
Step-by-step explanation:
The studies by the IAFF (International Association of Fire Fighters) and Orange County Fire Authority have shown that firefighters operating in full structural Personal Protective Equipment (PPE) will experience significant increases in core temperatures during twenty minutes of activity. Normal human body temperature averages around 98.6°F (37°C), and an increase of about 3°C is considered the maximum safe rise before heat stress or other heat-related conditions become a risk. Indeed, the question refers to a study involving core temperature elevation due to the thermal burden imposed by firefighting activities, which is consistent with the provided reference that a temperature increase greater than about 3°C would be considered unreasonably large. However, based on the information provided, there is not enough data to definitively select one of the given choices A, B, C, or D.