Almost there use socratic app
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CU is the element symbol for Copper.
<span>The correct answer is b. Radon. Oxygen, Hyrdrogen, and Boron, are not radioactivel ike Radon and it regenerates quickly meaning that even though it has a short half-life period, it stays for a long time once released. It also has no taste so it's difficult to notice without proper gear because you can't feel it.</span>
Here are the resonance contributors I found.
An example can be perspiration or even rain.
Hope this helps.
