You execute the following command in your Supportcenter.cn production database to change the width of the CUST_NAME column of the CUSTOMERS table：  ALTER TABLE customers  
MODIFY （cust_nameVARCHAR2 （40））  
/  
When you execute the command， it displays the following error message：  
ERROR at line 1：  
ORA-00054： resource busy and acquire with NOWAIT specified  
What could be the reason for the error message？（）

A.The database instance is not available.
B.The ALTER TABLE command does not have WAIT option
C.The table or a row in the table is currently locked by another user session.
D.The database instance is busy processing other user sessions commands.
E.The CUSTOMERS table has no long running query active at the time when this request is made.
F.The server process executing the ALTER TABLE command is busy with another command execution