Answer: three very important kinds of things: coal, oil, and natural gas.
Explanation:
D is the answer. A, B,C are absurd. Doing a little word right there.
The two elements that produce background radiation on earth are Radon and Uranium. Airborne radon can decay on its own. Radon undergoes alpha decay to produce Polonium. Uranium naturally undergoes alpha decay to produce Thorium.
It is a heterogeneous mixture <span>
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