A Rock from the Moon Was Found to Contain 1

A rock from the moon was found to contain 1.0 * 10^-5 g of uranium-238 and 4.0*10^-6 g lead-206. All of the lead-206 came from the decay of uranium-238. The half-life for this decay is
4) 5 * 10⁹ years. How old is this rock?

A) 1 * 10⁶ years
B) 2* 10⁷ years
C) 4 * 10⁸ years
D) 3 *10⁹ years
E) 5 * 10¹⁰ years