A $10-\text { pole, } 50 \mathrm{~Hz}, 240 \mathrm{~V},$ single phase induction motor runs at $540 ~\mathrm{RPM}$ while driving rated load. The frequency of induced rotor currents due to backward field is
- $100 \mathrm{~Hz}$
- $95 \mathrm{~Hz}$
- $10 \mathrm{~Hz}$
- $5 \mathrm{~Hz}$