Natural Uranium is a mixture of $$^{238}_{92}\text{U}$$ and $$^{235}_{92}\text{U}$$ with a relative mass abundance of $$140 \colon 1$$. The ratio of radioactivity contributed by the two isotopes of natural uranium, if their half-lives are $$4.5\times 10^{9}$$ years and $$7.0\times 10^{8}$$ years respectively, is

A

$$99.3 \colon 0.7$$

B

$$50.3 \colon 49.7$$

C

$$95.6 \colon 04.4$$

D

cannot be estimated