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: a light bulb is used for seeing in the dark. 
Sentence2: light is required for seeing.

A: seeing
****
Q: Sentence1: Lymphocytes are the key cells involved in the immune response. 
Sentence2: The key cells involved in the immune response produce antibodies.

A: cells
****
Q: Sentence1: ACID RAIN Acid rain is polluted and harmful to the environment. 
Sentence2: The emission of sulfur dioxide causes pollution to the environment.

A:
environment
****