A.EXECUTEDBMS_SCHEDULER.PURGE_LOG（log_history=>5，job_name=>’JOB1’）；
B.EXECUTEDBMS_SCHEDULER.PURGE_LOG（log_history=>5，job_name=>’JOB_LOG’）；
C.EXECUTEDBMS_SCHEDULER.PURGE_LOG（log_history=>5，which_log=>’JOB1’）；
D.EXECUTEDBMS_SCHEDULER.PURGE_LOG（log_history=>5，which_log=>’JOB_LOG’）；