**An igneous rock contains a radioactive isotope that has a half-life of 10 million years. Careful analysis shows that only one quarter of the original concentration of the parent isotope is left. How old is this igneous rock?**
A) 10 million years
B) 20 million years
C) 30 million years
D) 40 million years

Correct Answer: B – 20 million years

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