0 one of the many very small air bags in the lungs, with thin walls that allow oxygen to enter the blood -- 肺泡(肺中的细小气囊,细胞壁薄,有助于氧气进入血液)
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。