$A$ transformer is used to light a $100 \,W$ and $110 \,V$ lamp from a $220 \,V$ mains. If the main current is $0.5 \,A$, the efficiency of the transformer is approximately (in $\%$)
- A$30$
- B$50$
- C$90$
- D$10$
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The output power in a step-up transformer used in practice is . . . . . .
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