In 1972, a worker at a nuclear fuel plant in France found that uranium from a mine in Oklo, in the African Republic of Gabon, had less U-235 than the normal 0.7% a quantity known from meteorites and Moon rocks to be constant throughout the solar system. Further analysis showed the presence of isotopes that would result from the decay of fission products. Scientists drew the remarkable conclusion that a natural nuclear fission reaction had occurred some 2 billion years ago, lasting for about 100,000 years. Water, mixing with rich uranium ore, provided the moderator that enabled the chain reaction. More significantly, U-235’s 700-My half-life means that 2 billion years ago there was a higher abundance of U-235 in natural uranium.

Given U-238’s 4.5-billion-year half-life, the percentage of U-235 in natural uranium 2 billion years ago was
a. about 1%.
b. about 4%.
c. about 10%.
d. nearly 100%.