Which of the following statements is true regarding the initialization parameter DIAGNOSTIC_DEST？（）   The default value is the value of the environment variable $ORACLE_HOME； if $ORACLE_HOME isn’t set  

• A、 then the default is set to $ORACLE_BASE  The default value is the value of the environment variable $ORACLE_BASE； if $ORACLE_BASE isn’t set，
• B、 then it is set to $ORACLE_HOME
• C、 DIAGNOSTIC_DEST is always equal to $ORACLE_HOME
• D、 DIAGNOSTIC_DEST is always equal to $ORACLE_BASE