re: To "Iranian student"
There are several versions for the word 'Zoroaster', i mostly use Zarathustra because those who do not know what that means would think of "Zoro" and it becomes ridiculous so I always use "Zarathustra" version. like this one:
http://www.livius.org/za-zn/zarathustra/zarathustra.htm
But it's exact the same Zoroaster, I think Zarathustra became usual name after Friedrich Nietzsche wrote the book "Thus Spoke Zarathustra".
My reason to use Zarathustra is to avoid that "zoro" thing.