You need to ensure that the database users should be able to use the various flashback query features in order to go back in time by four hours. What would you do to achieve this?（）
Aset SQL_TRACE=True
Bset UNDO_RETENTION=14400
Cset FAST_START_MTTR_TARGET=240
Dset LOG_CHECKPOINT_INTERVAL=240
Eset DB_FLASHBACK_RETENTION_TARGET=14400
Fset the RETENTION GUARANTEE clause for the undo tablespace
Which object may be used during query optimization to improve the performance for a subset of SELECT queries?（）
A、MQT
B、Trigger
C、Read Only Cursor
D、Stored Procedure
以昆虫为媒介而传播的疾病是（）
A、麻疹
B、戊型肝炎
C、艾滋病
D、流行性出血热
E、结核病
N是自然数集合，定义f：N→N，f（x）＝xmod3，则f是（ ）。
A、满射不是单射
B、单射不是满射
C、双射
D、不是单射也不是满射
如果文件系统中有两个文件重名，不应采用（）。
A、单级目录结构
B、树型目录结构
C、二级目录结构
D、A和C
A UDT is a data type which:（）
A、must contain unique values.
B、cannot be CAST to other data types.
C、is created using the CREATE DISTINCT command.
D、contains a value that was automatically calculated from values in other columns.