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Incandescent bulbs are designed by keeping in mind that the resistance of their filament increases with the increase in temperature. If at room temperature, $100 \mathrm{~W}, 60 \mathrm{~W}$ and $40 \mathrm{~W}$ bulbs have filament resistances $\mathrm{R}_{100}, \mathrm{R}_{60}$ and $\mathrm{R}_{40}$, respectively, the relation between these resistances is
  • A$\frac{1}{R_{100}}=\frac{1}{R_{40}}+\frac{1}{R_{60}}$
  • B$\mathrm{R}_{100}=\mathrm{R}_{40}+\mathrm{R}_{60}$
  • C$\mathrm{R}_{100}>\mathrm{R}_{60}>\mathrm{R}_{40}$
  • D$\frac{1}{R_{100}}>\frac{1}{R_{60}}>\frac{1}{R_{40}}$
IIT 2010, Medium
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