The answer is high pressure. In a nuclear reaction process, high temperature and high pressure is required for turning hydrogen nuclei into a helium nucleus. It is impossible for it to happen simultaneously because it requires tremendous amounts of energy. Extreme conditions are required for the atoms to overcome electrostatic repulsion.
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Explanation:
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1) x=9, because we can use the Pythagorean theorem, x^2 + 144 = 225, x^2 = 81
2)
, because 6^2 +6^2 = 72, 
3)
, because 
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It’s d
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