You are performing a​ "stand-by" along with the fire department at a local county fair. Since it is fall and the temperature is​ cool, the emergency crews are sitting in their ambulances. Given this​ situation, what would contribute to an increase in the carbon monoxide levels in your​ ambulance?
A. Being parked too close to a fire engine that is also running
B. Having an exhaust system that discharges beyond the side of the ambulance
C. Keeping the heater running while you are inside the ambulance
D. Having had the ambulance​ tuned-up a week prior