Examine this syntax which allocates channels during a parallel image copy：  
RMAN> CONFIGURE DEVICE TYPE disk parallelism 4： 
2> COPY 
3> datafile 1 TO ‘/BACKUP/datafile1.dbf’， 
4> datafile 2 TO ‘/BACKUP/datafile2.dbf’， 
5> datafile 3 TO ‘/BACKUP/datafile3.dbf； 
RMAN>COPY 
2> datafile 4 TO ‘/BACKUP/datafile4.dbf’；  
How many channels are used？（）