Your tnsnames.ora file has the following entry for the service alias ORCL：  ORCL =  （DESCRIPTION =  （ADDRESS = （PROTOCOL = TCP） （HOST = 10.156.24.216） （PORT = 1521））   （CONNECT_DATA =  （SERVER = DEDICATED） （SERVICE_NAME = orcl.oracle.com）   ）   ）   The TNSPING command executes successfully when tested with ORCL but you are not able to connect to the database instance with the following command：  SQL> CONNECT scott/tiger@orcl  What could be the reason for this？（）

• A、The listener is not running at the server.
• B、The TNS_ADMIN environmental variable is set to a wrong value.
• C、The orcl.oracle.com database service is not registered with the listener.
• D、The DEFAULT_DOMAIN parameter is set to a wrong value in the SQLNET.ORA file.