When is carbon monoxide most likely to leak into a vehicle?

• When the heater is running & exhaust system is not working
• In heavy traffic where exhaust fumes are breathed in from other vehicles
• When the engine is turned off
• Both A & B
  Explanation:

  "Carbon monoxide gas from a vehicle engine can harm or kill you or your passengers. Carbon monoxide is most likely to leak into a vehicle when its heater is running, when the exhaust system is not working properly, or in heavy traffic where exhaust fumes are breathed in from other vehicles. A faulty exhaust system can leak poisonous fumes into a vehicle’s back seat."