A single-phase transformer has no-load loss of $64W$,as obtained from an open-circuit test.when a short-circuit test is performed on it with $90\%$ of the rated current flowing in its both $LV$ and $HV$ winding,the measured loss is $81W$.The transformer has maximum efficiency when operated at
1. $50.0 \:\%$ of the rated current.
2. $64.0 \:\%$ of the rated current.
3. $80.0\: \%$ of the rated current.
4. $88.8 \:\%$ of the rated current.