Two long parallel wires, AB and CD, carry equal currents in opposite directions. They lie in the xy plane, parallel to the x-axis, and pass through the points (0, –1, 0) and (0, 1, 0) respectively. The resultant magnetic field is

• A

  zero on the x-axis

• B

  maximum on the x-axis

• C

  directed along the z-axis at the origin, but not at other points on the z-axis

• D

  none of these