The correct answer is: [D]: "S" (sulfur).
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Note: The other answer choices:
[A]: "K" (potassium) ;
[B]: "Na" (sodium) ;
[C] "Mg" (magnesium) ;
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are all metals.
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Answer choice: [D]: "S" (sulfur) ; is NOT a metal; and may exist in gaseous form.
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