A radio isotope 'X' with a half life 1.4 10 9 years decays to 'Y' which is stable. A sample of the rock from a cave was found to contain 'X' and 'Y' in the ratio 1:7 . The age of the rock is :

1. A. 1.96 10 9 years
2. B. 3.92 10 9 years
3. C. 4.20 10 9 years
4. D. 8.40 10 9 years

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