Consider an electron microscope shooting electron electrons

Consider an electron microscope shooting electron electrons at 0.5c towards a target. The microscope is in an external magnetic field of 2.4*10^-4 T that is creating problems for this lab. Consequently it is necessary to produce a magnetic field outside the microscope, in order to cancel this interfering field. In the picture the beam goes down (-z direction). The x direction is straight out of the screen towards you. The external field is in that direction.

The lab considers installing a wire that will carry a large enough current to make a magnetic field that cancels the interfering field at the target. Which way would it be possible for the field of the wire to cancel the interfering field.

A: along the x-axis

B: Along the Z-axis

C:along the z or y axis ,but not the x axis.

The answer is C but I am looking for a short explanation why it is C

Solution

According to the given problem,

The answer is C : Along z or y axis, but not x axis

Reason: As it is a stright wire and causing the field in x direction to cancle that an electric field must be oriented perpendicular to the given axis to cancel it.