From the calculation, there is still more than 75 percent on the earth now (option D).
The term half life is the time taken for half of the number of radioactive isotopes to remain.
Given that;
N/No = (1/2)^t/t1/2
N = amount at time t
No = amount at the beginning
t = time taken
t1/2 = half life
Hence;
N/No = (0.5)^4.6 * 10^9/50 * 10^9
N/No = 0.94
Hence, the percentage that is currently in the earth is about 94%.
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