0 used when the subject of the verb is "they" or a group of people, and the object is the same group of people -- (they 的反身形式)他們自己,她們自己,它們自己
We can complete it step by step however long the road is and it can't be completed however short the road is if you don't even mark your footprint.
再长的路,一步步也能走完,再短的路,不迈开双脚也无法到达。