How to enter Control + alt + delete in vitual box from mac

On an OS X host, try
  • control+alt/option+fn+delete or
  • cmd+fn+delete
I used the onscreen keyboard in Windows 7 which showed the guest OS receiving ctrl and alt/option, but not delete. By adding the fn key to the combination, it seems the Mac's idea of "delete" (normally like Windows' "backspace") becomes Windows' "delete".