su doesn't know '-g' or '--group'

[hanpingtian]

The su --help give this messages:
Usage: su [OPTION]... [-] [USER [ARG]...]
Change the effective user id and group id to that of USER.

 -, -l, --login               make the shell a login shell
 -g --group=group             specify the primary group
 -G --supp-group=group        specify a supplemental group
 -c, --commmand=COMMAND       pass a single COMMAND to the shell with -c
 --session-command=COMMAND    pass a single COMMAND to the shell with -c
                              and do not create a new session
 -f, --fast                   pass -f to the shell (for csh or tcsh)
 -m, --preserve-environment   do not reset environment variables
 -p                           same as -m
 -s, --shell=SHELL            run SHELL if /etc/shells allows it
     --help     display this help and exit
     --version  output version information and exit

A mere - implies -l.   If USER not given, assume root.
but when try to run su with '--group', it says:
su: unrecognized option `--group'
Try `su --help' for more information.

su --version
su (GNU coreutils) 5.97
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software.  You may redistribute copies of it under the terms of
the GNU General Public License <http://www.gnu.org/licenses/gpl.html>.
There is NO WARRANTY, to the extent permitted by law.

Written by David MacKenzie.
I using fedora core 6.