[CentOS] 'p' in boot selector & going into single-user

On boot, when I hit the F12 key to bring up the boot menu, the screen
displays the various kernels and other boot options.  In the text below
this menu it says I can hit 'p' to bring up a menu with other options.
I've done this with the last two boot-ups and no other menu or options
appear.  Is something missing?  (I also tried Alt-p, Shift-p, Ctrl-p and
a few other keys and there's no change onscreen.)


I'd like to know also how to select single user mode, but haven't found
a way to do that.

Thanks much.