The thermo $e.m.f.$ of a thermocouple is $25\,\mu V/^{\circ}C$ at room temperature. $A$ galvanometer of $40\,\Omega$ resistance, capable of detecting current as low as $10^{-5}\,A$, is connected with the thermocouple. The smallest temperature difference that can be detected by this system is ................ $^{\circ}C$.

  • A
    $20$
  • B
    $16$
  • C
    $12$
  • D
    $8$

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