A cell sends a current through a resistance $R$ for time $t$. Now the same cell sends current through another resistance $r$ for the same time. If same amount of heat is developed in both the resistance, then the internal resistance of cell is
  • A$\frac{(R+r)}{2}$
  • B$\frac{(R-r)}{2}$
  • C$\frac{(R r)}{2}$
  • D$\sqrt{(R r)}$
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