How to choose a carbon monoxide detector with This Old House

Watch this video from This Old House to learn how to choose a carbon monoxide detector.
Steps:

1 Simplest, most affordable carbon-monoxide detector plugs into an electrical outlet.
2 Battery-powered plug-in models continue to operate during power outages.
3 Combination units detect smoke and carbon monoxide, and are hard-wired into the home's electrical system.
4 Unlike smoke detectors, which must be ceiling-mounted,
carbon-monoxide detectors can be mounted to a baseboard or set on a bookshelf.
5 Digital models constantly display the carbon monoxide levels in the room.
6 Recommendation is to place one carbon-monoxide detector on every floor in the home; minimal requirement is one detector outside of every bedroom.