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