The average surface area of a reservoir in the month of June is $20 \: km^2$. In the same month, the average rate of inflow is $10 \: m^3/s$, outflow rate is $15 \: m^3/s$, monthly rainfall is $10$ cm, monthly seepage loss is $1.8$ cm and the storage change is $16 \: million \: m^3$. The evaporation (in cm) in that month is:

1. $46.8$
2. $136.0$
3. $13.6$
4. $23.4$