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# 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

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

The point $(0, 0, -a)$ lies on the $z$ - axis $\therefore$ the field due to the $z$ - current is zero.
The field due to the other wires can be found out by using the right hand rule
Ans : (b)