What best describes an FXO interface？（）
- Aanalog trunks that provide the Survivable Remote Site Telephony (SRST) feature
- Banalog trunks that provide VoIP gateway functionality
- Canalog trunks that connect a gateway to plain old telephone service (POTS) device such as analog phones, fax machines, and legacy voice-mail systems
- Danalog trunks that connect a gateway to a central office (CO) or private branch exchange (PBX)