The Decay of ⁴⁰K to ⁴⁰Ar Can Be Used to Determine

The decay of ⁴⁰K to ⁴⁰Ar can be used to determine the age of very old rocks. If moon rocks from the Sea of Tranquility have 12.5% of the estimated original amount of ⁴⁰K, approximately how old are the rocks? ⁴⁰K decays with a rate constant of 5.06 x 10^-10 yr^-1.

A) between 10⁶ and 10⁷ years
B) between 10⁸ and 10⁹ years
C) between 10¹⁰ and 10¹¹ years
D) between 10¹² and 10¹³ years
E) more than 10¹³ years