Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

[Q]: Sentence1: the emission of sulfur dioxide causes acid rain. 
Sentence2: Sulfur dioxide emissions causes soil nutrients to be leached away.
[A]: dioxide


[Q]: Sentence1: Limestone caves displaying stalactites. 
Sentence2: Limestone caves are formed by depositions.
[A]: Limestone


[Q]: Sentence1: a valley is formed by a river flowing. 
Sentence2: Valleys are formed by fish habitats.
[A]:
formed