0 a substance that can cause an allergy (= condition of the body reacting badly to something) but is not harmful to most people -- (引起過敏反應的)變應原;過敏原
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。