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.
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Question: Sentence1: a fire requires oxygen to burn. 
Sentence2: If the oxygen is removed, the fire is smothered.

Answer: oxygen


Question: Sentence1: Bivalves feed on plankton and nonliving organic matter. 
Sentence2: Clams feed on plankton and nonliving organic matter.

Answer: feed


Question: Sentence1: Acid rain can harm plant life and actually kill plants. 
Sentence2: gases from burning coal and oil dissolving in water can harm plant life.

Answer:
harm