Task: Sentence Ordering
Input (English): Steps: (1) The pollution negatively affects the water quality (2) The pollution negatively affects the health of plants and animals (3) The plants and animals have shorter, less healthy lives (4) The plants and animals do not reproduce very well (5) The number of plants and animals decreases over time (6) The quality of water gets worse and worse as there are no plants or animals to clean it (7) Eventually there is nothing living in the water and it is unusable  
Where does the following step fit into the given list of steps: Pollution enters natural water sources such as rivers and lakes 
Output (English):  Before step 1.