A battery of $e.m.f.$ $10\, V$ and internal resistance $0.5\, ohm$ is connected across a variable resistance $R$. The value of $R$ for which the power delivered in it is maximum is given by ......... $ohm$
A$2$
B$0.25$
C$1$
D$0.5$
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D$0.5$
d For maximum power $r = R$
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