In order to use the user and group mappings stored in an LDAP directory, which setting in smb.conf is correct?

A.
idmap backend = ldap://ldap.example.com:636

B.
idmap backend = ldap:ldap://ldap.example.com:636

C.
idmap backend = ldap server = ldap://ldap.example.com:636

D.
idmap backend = ldap