Task: Sentence Ordering
Input (English): Steps: (1) Limestone is located under the soil (2) Rain picks up carbon dioxide as it falls to earth (3) The carbon dioxide in the rain washes through the soil (4) The carbon dioxide turns into acid (5) The acid in the rain gets to the limestone below the soil (6) The acid dissolves the limestone (7) Acid continues to erode the limestone with more rain over time (8) The eroded limestone sometimes forms caves.  
Where does the following step fit into the given list of steps: The rain falls on the soil over the limestone 
Output (English):  After step 2.