Answer: 2) "sodium" .
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(or, " Na " ; as it appears on the Periodic Table of Elements).
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Note:
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Answer choice: 1) carbon dioxide ; or, " CO₂ " ; is a compound of TWO elements.
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Answer choice: 3) ammonia ; or, " NH₃ " ; is a compound of TWO or more elements; and is often found in a solution, or "mixture.
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Answer choice: 4) sand — variety of types — is a mixture of more than one elements; or most often, either: 1) a mixture of compounds; or 2) a mixture of various compounds and elements.
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Answer choice: 2) sodium ; or, " Na " ; is a "single element".
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