$500 \: \mathrm{mg}$ of a dry adsorbent is added to a beaker containing $100 \: \mathrm{mL}$ solution of concentration $100 \: \mathrm{mg \: phenol/ (L \: solution)}$. The adsorbent is separated out after $5 \: \mathrm{h}$ of rigorous mixing. If the residual concentration in the solution after separating the adsorbent is $30 \: \mathrm{mg \: phenol/(L \: solution)}$, the amount of phenol adsorbed (in mg per gram of dry adsorbent) is
- $7$
- $14$
- $28$
- $18$
