You find a rock containing radioactive potassium-40 and its decay product argon-40. You assume that all the argon-40 was made from radioactive decay of potassium-40. The rock now has twice as much argon-40 as potassium-40; that is, 2/3 of the original potassium-40 has decayed into argon-40, whereas 1/3 remains in the rock. Based on this graph, about how old is the rock?
A) 1 billion years
B) 1.25 billion years
C) 2 billion years
D) 2.5 billion years
C) 2 billion years
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