A Rock Contains 0 $\rarr$ ²⁰⁶Pb Is 4

A rock contains 0.37 mg of Pb-206 and 0.95 mg of U-238. Approximately how many U-238 atoms were in the rock when it was formed billions of years ago (The half life for ²³⁸U $\rarr$ ²⁰⁶Pb is 4.5 * 10⁹ yr.)

A) 1.32 atoms
B) 5.8 * 10^-6 atoms
C) 2.4 * 10¹⁸ atoms
D) 3.5 * 10¹⁸ atoms
E) 3.5 * 10²¹ atoms