A single-phase transformer has maximum efficiency of $98 \%$. The core losses are $\text{80 W}$ and the equivalent winding resistance as seen from the primary side is $0.5 \:\Omega$. The rated current on the primary side is $\text{25 A}$. The percentage of the rated input current at which the maximum efficiency occurs is
- $35.7\%$
- $50.6\%$
- $80.5\%$
- $100\%$