Answer
U-238, $t_{\frac{1}{2}} = 4.5*10^{9} years$ and Th-232, $t_{\frac{1}{2}} = 1.4*10^{9} years$
They are still present because of their long half-lives.
Work Step by Step
U-238, $t_{\frac{1}{2}} = 4.5*10^{9} years$ and Th-232, $t_{\frac{1}{2}} = 1.4*10^{9} years$
They are still present because of their long half-lives.
