The Atom ⁴⁰K Decays Radioactively to the Gas ⁴⁰Ar

Multiple Choice

The atom ⁴⁰K decays radioactively to the gas ⁴⁰Ar.An ancient rock is found to contain enough ⁴⁰Ar gas to indicate that 77.0% of the ⁴⁰K in the rock has decayed to ⁴⁰Ar since the rock solidified.(Any previous argon would have boiled out of liquid rock. ) How long ago did the rock solidify? The half-life of ⁴⁰K is 1.25 billion years.

A) 2.65 billion years
B) 0.471 billion years
C) 1.80 billion years
D) 0.324 billion years