Acme Company has offices around the country. With the use of Cisco Unified CallManager， the company has deployed a VoIP network in a multisite centralized configuration. Which IOS gateway feature should be deployed to enable VoIP devices to register with a local gateway and continue to function when the connection to the Cisco Unified CallManager is broken？ （）

• A、gatekeeper multizone
• B、Call Admission Control （CAC）
• C、Automatic Alternate Routing （AAR）
• D、Survivable Remote Site Telephony （SRST）