A river sample was analyzed for Cd$2+$ and Pb$2+$ using GFAA. $25\mathrm{.}0$ microliters of river sample gave an absorption signal for Cd$2+$ of $0\mathrm{.}123$ and Pb$2+$ of $0\mathrm{.}159.$ To the river sample was added a standard stock solution containing $15\mathrm{.}0$ micrograms per liter Cd$2+$ and $20\mathrm{.}0$ micrograms per liter Pb$2+.$ If $5\mathrm{.}00$ microliters of standard was added to $20\mathrm{.}0$ microliters of river water and the absorption signals were now measured at Cd$2+0\mathrm{.}225$ and Pb$2+0\mathrm{.}302,$ what is the ppb concentration of Cd$2+$ in the river sample?

Question $11$ options:

$3\mathrm{.}16$ ppb

$5\mathrm{.}19$ ppb

$2\mathrm{.}91$ ppb

$8\mathrm{.}16$ ppb

$3\mathrm{.}95$ ppb