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Which of the following is not an example of a pure element?
Ar
Copper wire
CO{2}
O{2}

 copper wire

Explanation:

Copper wire is not an example of a pure element because although it's made by pure elements, it's not one itself.

O2

Explanation:

Co2 is a pure mixture while O2 is a substance, but oxygen by itself is an element. Copper in any form, wire, bar, tube is still an element. Shape does not affect an element being an element.

Ar is a pure element.