Which of the following are valid choices for the LDAP database backend? (Choose THREE correct answers.)

'file' is not a standard, standalone OpenLDAP database backend; data is typically stored in specialized formats managed by other backends.

The 'config' backend is a special database that stores the runtime configuration of OpenLDAP, allowing dynamic management of the server's settings.

The 'passwd' backend allows OpenLDAP to serve directory information directly from the local system's '/etc/passwd' and '/etc/group' files, integrating local user and group data.

'lbdm' is not a recognized or standard database backend option for OpenLDAP.

The 'shell' backend provides a mechanism for OpenLDAP to interact with external programs or scripts to retrieve and manage directory data, offering flexibility for custom data sources.