The data for an agricultural field for a specific month are given below:
$\begin{array}{ll}\text{Pan Evaporation} &=\:100\:mm\\ \text{Effective Rainfall} &=\:20\:mm\;\text{(after deducting losses due to runoff and deep percolation)} \\ \text{Crop Coefficient} &=\:0.4\\ \text{Irrigation Efficiency} &=\:0.5\end{array}$
The amount of irrigation water (in $mm$) to be applied to the field in that month, is
- $0$
- $20$
- $40$
- $80$