Solubility product of silver bromide is $5.0\times 10^{-13}$. The quantity of potassium bromide (molar mass taken as $120{\text{g mol}}^{-1}$) to be added to $1\text{litre}$ of $0.05\text{M}$ solution of silver nitrate to start the precipitation of $\text{AgBr}$ is:

1. A
   $1.2\times 10^{-10}\text{g}$
2. B
   $1.2\times 10^{-9}\text{g}$
3. C
   $6.2\times 10^{-5}\text{g}$
4. D
   $5.0\times 10^{-8}\text{g}$