A sample from a potable water system in a small city is analyzed and the results report $20m\frac{g}{L}M{g}^{++}$and $50m\frac{g}{L}C{a}^{++}.$ The hardness of the water expressed in $m\frac{g}{L}$ as $CaC{O}_3$ is most nearly:

$($A$)156m\frac{g}{L}$ as $CaC{O}_3$

$($B$)207m\frac{g}{L}$ as $CaC{O}_3$

$($C$)286m\frac{g}{L}$ as $CaC{O}_3$

$($D$)324m\frac{g}{L}$ as $CaC{O}_3$