Which two statements about the background process of thedatabase writerare true（） 

• A、It is possible to have multiple database writers in an Oracle instance.
• B、It writes dirty buffers， if any， to data files whenever a checkpoint occurs.
• C、It writes dirty buffers， if any， to data files whenever a transaction commits.
• D、It writes dirty buffers， if any， to data files before the log writer （LGWR） writes.