The correct statement is:
D. O2 is a molecule while CO2 is a compound.
Here's the reasoning:
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O2 is a molecule because it is made up of two oxygen atoms bonded together. However, since both atoms are of the same element, it is not considered a compound.
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CO2 is a compound because it consists of two different elements (carbon and oxygen) bonded together.
Thus, option D correctly identifies O2 as a molecule and CO2 as a compound.