Why has nuclear fusion been difficult to develop?
A. It is hard to recreate the extremely high temperatures and pressures found inside stars.
B. An efficient shape to contain the reaction has not been found.
C. Fusion normally occurs in the vacuum of space and the atmosphere on Earth stops the reaction.
D. Fuel for nuclear fusion is very rare.

Where is the graph ?

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Radon-222 or rn-222 has a half-life of 3.82 days. to determine the fraction of an original sample of rn-222 that remains unchanged after 7.646 days, we calculate the amount of rn-222 remaining after 7.646 days and divide it with the original amount. using 1 gram as a basis and using half-life calculation methods, we can determine that  0.249 or 0.25  grams of rn-222 will remain, and thus the fraction remaining is 0.25 or 1/4.