BRK First Alert SC9120B Hardwire Combination Smoke/Carbon Monoxide Alarm with Battery Backup 4 Pack Single button test/silence 9V Alkaline Battery Backup Latching alarm indicator - remembers which unit initiated an alarm Two silence features - can silence the low-battery chirp for up to eight hours or temporarily silence an unwanted nuisance alarm End of Life Signal provides audible notification alarms needs to be replaced Intelligent Sensing TechnologyTM helps reduce the number of nuisance alarms Perfect Mount - allows for easy alignment of alarm 7-Years Limited Warranty Easy Installation/Maintenance - Preferred features include a large opening in the mounting bracket for easy access to wiring. Keyhole slots in the mounting bracket elimin