Two parallel wires carrying equal currents in the opposite directions are placed at $x = \pm$ a parallel to the $y$-axis with $z = 0$. Magnetic field at origin $O$ is $B1$ and at $P (2a,0,0)$ is $B2$. Then the ratio $\large\frac{B1}{B2}$ is

Question

$\begin {array} {1 1} (a)\;-3 & \quad (b)\;-\large\frac{1}{3} \\ (c)\;-\large\frac{1}{2} & \quad (d)\;2 \end {array}$