Which statement is true about Foreign Exchange Station （FXS） ports on a router？（）
A. The FXS interface allows an analog connection to be directed at the public switched telephone network （PSTN’s） central office.
B. The FXS interface connects directly to a standard telephone，fax machine，or similar device and supplies ring，voltage，and dial tone.
C. The FXS interface connects directly to an IP phone and supplies ring，voltage，and dial tone.
D. The FXS interface connects directly to ISDN voice channels.
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