A uniform wire of resistance $20\,ohm$ having resistance $1\Omega /m$ is bent in the adjoining form of a circle as shown in the figure. If the equivalent resistance between $M$ and $N$ is $1.8 \,\Omega $, then the length of the shorter section is ................ $m$
  • A$2$
  • B$5$
  • C$1.8$
  • D$18$
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