Solute $C$ is extracted in a batch process from its homogenous solution of $A$ and $C$, using solvent $B$. The combined composition of the feed and the extracting solvent is shown in the figure below as point $M$, along with the tie line passing through it. The ends of the tie line are on the equilibrium curve.

What is the selectivity for $C$?

1. $3.5$
2. $7$
3. $10.5$
4. $21$

Answer: