A.RMAN will attempt to make the backup run as fast as possible without any IO limitations. 
B.RMAN will automatically restrict the number of channels in use to one. 
C.RMAN will spread the backup IO over the total duration stated in the backup command.
D.RMAN will skip any datafile that currently is involved in an IO operation. RMAN will retry backing up the datafile later and an error will be raised at the end of the backup if the datafile cannot be backed up. 
E.Datafiles will be backed up; those having the lowest current number of IO operations will be backed up first.