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

1. $0$
2. $20$
3. $40$
4. $80$
in Others
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