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.