Q. Argon is the third most common gas in the Earth’s atmosphere, at 0.93%. This gas is generated due to decay of which among the following isotopes in the Earth’s crust ?
Answer: K-40
Notes: Potassium-40 is a radioactive isotope of potassium with a long half-life of 1.251×10⁹ years. The decay of this particular isotope explains the large abundance of argon (0.93%) in the Earth's atmosphere.

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