Universal Security Instruments MCND401B M Series Plug-In Carbon Monoxide and Natural Gas Alarm with 9-Volt Battery Backup *4-Pack* Large, Bright Digital Display Screen with Three LED Colors to Indicate Status of Power, Carbon Monoxide and Natural Gas Ideal for Every Room In Your Home with a Compact Design - Covers Only One Outlet Fast and Easy to Install - Plugs In to Standard Outlet: No Additional Mounting Required 9-Volt Energizer Alkaline Battery with Quick Activation Pull Tab Included - Provides Protection Even During a Power Outage Microprocessor Software Controlled - Virtually Eliminates Nuisance Alarms