The half-life of U235 is 7 x 10^8 years and it decays into Pb207. If you have a 1 rock sample containing minerals having 1 unit of U235 and 3 units of Pb207, how old is the rock sample? a. 2.8 x 10^9 years b. 1.4 x 10^9 years c. 2.1 x 10^9years d. 7 x 10^8 years