A cluster indicates that a remote site has failed. What facilities exist in HACMP/XD://HAGeo to  ensure that the failure is not just a TCP/IP communication failure？（）  

• A、 Electronic Service Agent software provides a redundant heartbeat between sites.
• B、 A non-IP network and Dial Back Fail Safe provide redundant heartbeats between sites.
• C、 The GeoMirror Devices provide a link validation facility that guarantees the validity between sites.
• D、 The Site Notification Daemon maintains a heartbeat between sites via short haul Modems/line drivers and dark fibre.