Answer: Radon
Explanation: Radium is a decay product of Uranium. As Radium is still highly radioactive it further decays into Radon.
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Answer: carbon dioxide that enters through the leaf openings
Explanation:
Answer: Solid
Explanation: The rest of the options don't have a definite shape nor volume.
Most carbon atoms are carbon-12, and these have 6 neutrons, but carbon-14 has 8 neutrons (2 extra), thus C-14 is considered an isotope of the element carbon.