I tried to modify the sudoers file on Raspbian ` /etc/sudoers`, and it looks like I made a mistake, as I'm unable to modify it again. I am also unable to call any command that requires `sudo` permissions.
So I have done some research and I found a solution <a href="http://askubuntu.com/questions/73864/how-to-modify-a-invalid-etc-sudoers-file-it-throws-out-an-error-and-not-allowi ">here</a>.

The solution uses the command `*pkexec visudo*`, but when I tried that it gives me an error message: `Authentication is needed to run '/usr/sbin/visudo' as the super user`. Also there is a drop-down list, it contains only the word 'root' (and couldn't be changed). As well as named Identity and a blank text-box named Password.

So the question is how could I know the root password? I tried 'root', 'raspberry', 'admin' e.t.c... but nothing has worked.