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The correct answer is Option B — 1, 3, 9, 27, 81... In an uncontrolled chain reaction (such as in an atom bomb), when U-235 absorbs a neutron and undergoes fission, it releases approximately 3 neutrons. Each of these 3 neutrons causes another fission releasing 3 more neutrons each, so after 2nd generation: 9 neutrons, 3rd: 27, and so on. This is a geometric progression with ratio 3. Option A is incorrect: The sequence 1, 2, 4, 8 represents doubling (ratio 2) which would occur if each fission released only 2 neutrons. While some reactions release 2 neutrons, the standard value for U-235 is approximately 2.5, rounded to 3 for chain reaction calculations. Option C is incorrect: The ratio 5 (1, 5, 25, 125) is incorrect as U-235 fission does not release 5 neutrons per event. Option D is incorrect: Ratio 4 (1, 4, 16, 64) is also not the correct neutron multiplication for U-235 fission. The rapid exponential growth in neutron numbers causes the explosive energy release in an atom bomb.
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