An Ancient Rock Is Found to Contain ⁴⁰Ar Gas,indicating That

An ancient rock is found to contain ⁴⁰Ar gas,indicating that of the ⁴⁰K in the rock has decayed since the rock solidified.Any argon would have boiled out of liquid rock.The half-life of ⁴⁰K is 1.25 billion years.How long ago did the rock solidify?

A) 2.6 billion years
B) 0.50 billion years
C) 1.8 billion years
D) 0.30 billion years