a
(a)\(l = \frac{{FL}}{{\pi {r^2}r}}\)==> \(l \propto \frac{F}{{{r^2}}}\) (Y and L are constant) \(\frac{{{l_2}}}{{{l_1}}} = \frac{{{F_2}}}{{{F_1}}} \times {\left( {\frac{{{r_1}}}{{{r_2}}}} \right)^2} = 2 \times {(2)^2} = 8\)
\({l_2} = 8{l_1} = 8 \times 1 = 8mm\)