A steady current $I$ is flowing in the $-x$ direction through each of two infinitely long wires at $y = ± \frac{L}{2}$ as shown in the figure. The permeability of the medium is $\mu_{0}$. The $\vec{B}$-field at $(0,L,0)$ is

1. $-\frac{4\mu _{0}I}{3\Pi L} \hat{Z}$
2. $+\frac{4\mu _{0}I}{3\Pi L} \hat{Z}$
3. $0$
4. $-\frac{3\mu _{0}I}{4\Pi L} \hat{Z}$

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