Same current is flowing in three infinitely long wires along positive $x, y$ and $z$ directions. The magnetic field at the point $(0, 0, -a)$ would be

Question

$\begin {array} {1 1} (a)\;\large\frac{\mu_0i}{2 \pi a}( \hat i - \hat j) & \quad (b)\;\large\frac{\mu_0i}{2 \pi a}( \hat j - \hat i) \\ (c)\;\large\frac{\mu_0i}{2 \pi a}( \hat i + \hat j) & \quad (d)\;\large\frac{\mu_0i}{2 \pi a}( \hat i + \hat j+ \hat k) \end {array}$