View the Exhibit. 

You executed the following command to perform a backup of the USERS tablespace： RMAN> BACKUP TABLESPACE USERS; 
Which type of backup would this command perform？（）

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： 
RMAN­03009： failure of allocate command on c1 channel 
ORA­19554： error allocating device， device type： SBT_TAPE， device name： 
What could explain why the statement failed？（）