Is there a difference between steunen and ondersteunen ? I was wondering why the alternative with ondersteunen cannot be considered correct.
There is a difference. Steunen usually doesn't refer to physical support, but rather mental support, ondersteunen is either physical support or to help/aid (usually by giving money).
Hm...the translation above does not match the one on the page, "De argumenten steunen het opvatting niet." The above translation apeears to be correction, while the one on the actual page is wrong.