You execute the following command to perform a backup by using Recovery Manager （RMAN）： RMAN> run  {allocate channel c1 type sbt;  backup datafile 1;  }  The command errors out. A part of the error message is as follows：  RMAN/x7f03009： failure of allocate command on c1 channel  ORA/x7f19554： error allocating device， device type： SBT_TAPE， device name：  What could explain why the statement failed？（）

• A、sbt is not a valid device type.
• B、Media Management Library was not loaded.
• C、The sbt device cannot be used for data file backups.
• D、No path was defined in the ALLOCATE CHANNEL command.
• E、The channel is currently being used by another RMAN session.