The magnetic field a distance from an infinite current loop carrying current is
This follows immediately from Ampere's Law. Therefore, the magnetic field at the leftmost wire will be
The force on the leftmost wire will therefore be
As we found in Problem 54, this force is directed to the left. The magnetic field at the rightmost wire will be
As we found in Problem 54, this force is directed to the right. The forces on the bottom and the top of the wire will cancel, because the magnetic fields are the same, but the currents are in opposite directions. Therefore, the net force on the wire is
Therefore, answer (D) is correct.