Today, Uranium Contains $0.72 \%$ 235U (Half-Life $= 0.70$ Billion Years) And $99.28 \% 238 \cup$

Multiple Choice

Today, uranium contains $0.72 \%$ 235U (half-life $= 0.70$ billion years) and $99.28 \% 238 \cup$ (half-life $=$ $4.5$ billion years) . At a time $1.9$ billion years ago, what was the fraction of $235 \mathrm { U }$ in uranium?

A) $3.53 \%$
B) $4.90 \%$
C) $6.76 \%$
D) $4.72 \%$