You don't have to put a copyright notice on the
questions in order to protect your copyrights. That
requirement doesn't apply to works published since 1989 (I
think that's the date). Nor do you have to register
them with the U.S. Copyright Office, although
registration allows you to sue in federal court, and allows
you to obtain statutory damages, which would be
especially useful in QB infringement cases where actual
damages will be minimal.
Generally speaking (I'm
at home, not work), to show infringement, you have
to show (1) that your work was original and
copyrightable, (2) substantial similarity of the two works, and
(3) that the alleged infringer had access to the
original work.
Chris Sloan (a lawyer who does
copyright work)