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> **Possible Duplicate:**  
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> [Are there any simple rules for article usage (“a” vs “the” vs none)](https://english.stackexchange.com/questions/2031/are-there-any-simple-rules-for-article-usage-a-vs-the-vs-none) 
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Which is correct?

1. Everyone knows what a proton is.
2. Everyone knows what the proton is.
3. Everyone knows what proton is.