A) 1.25\[m\]
B) 2.5\[m\]
C) 10\[m\]
D) 20\[m\]
Correct Answer: D
Solution :
\[R\propto \frac{l}{{{r}^{2}}}\] \[\Rightarrow \] \[\frac{{{R}_{1}}}{{{R}_{2}}}=\frac{{{l}_{1}}}{{{l}_{2}}}\times \frac{r_{2}^{2}}{r_{1}^{2}}\Rightarrow \frac{1}{1}=\frac{5}{{{l}_{2}}}\times {{\left( \frac{2}{1} \right)}^{2}}\Rightarrow {{l}_{2}}=20m\]
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