A Rock Sample from the Moon Includes a Mineral That

A rock sample from the moon includes a mineral that contains small amounts of the radioactive isotope Potassium- 40 and its daughter element Argon- 40 (half- life of 1.3 billion years) . This mineral would not form with any Argon- 40. Consider a crystal with 7 atoms of Argon- 40 for every 1 atom of Potassium- 40. How many half- lives has this sample existed?

A) 1
B) 2
C) 3
D) 4