c
$B = \frac{{{\mu _0}}}{{4\pi }}\frac{{\theta \,i}}{r} \Rightarrow B \propto \theta \,i$ ($\frac{{{i_1}}}{{{i_2}}} = \frac{{{l_2}}}{{{l_1}}} = \frac{{{\theta _2}}}{{{\theta _1}}}$)
$ \Rightarrow \frac{{{B_1}}}{{{B_2}}} = \frac{{{\theta _1}}}{{{\theta _2}}}.\frac{{{i_1}}}{{{i_2}}}$
$\frac{{{B_1}}}{{{B_2}}} = \frac{{{\theta _1}}}{{{\theta _2}}} \times \frac{{{\theta _2}}}{{{\theta _1}}}$
$ \Rightarrow {B_1} = {B_2}$
