The isotopes `.^(238)U` and `.^(235)U` occur in nature in the mass ratio 140 : 1. it is assumed that intially they were found in equal mass. If half life `(t_(1//2))` of `.^(238)U=4.5xx10^(9)` and `t_(1//2)` of `.^(235)Uxx10^(8)` years respectively then the age of earth is `(log 7=0.846,log2=0.3)`A. `4.02xx10^(9)` yearB. `2.01xx10^(9)` yearC. `8.72xx10^(9)` yearD. none of these