Please choose the correct ending to complete the given sentence: Health: [header] How to avoid carbon monoxide poisoning [title] Perform proper maintenance. [step] Your heating system (including chimneys, ducts, and vents) and your hot water heater should be thoroughly inspected by a qualified technician every year. [title] Keep generators outside.

Ending1: [step] Building electricity is expensive, and heavy plumbing pipes will take time to break down properly. Electricity companies will likely have stricter rules about how to safely operate the generators throughout the home. Ending2: [step] Never use a portable generator inside your home or garage, even if the doors and windows are open. Only use generators outside, at least 20 feet away from your doors and windows. Ending3: [step] The pipes leading to your thermostat (power supply) could be either too long or too short. [substeps] Call your service provider if you want to verify that a heat-source is plugged in and safe for lower temperatures. Ending4: [step] The next time you use your house, keep the generator in another room to prevent any contamination. [title] Understand the benefits of not adding carbon to the air.

Answer format: ending1/ending2/ending3/ending4