Two Long, Straight, Parallel Wires Separated by a Distance D

Two long, straight, parallel wires separated by a distance d carry currents in opposite directions as shown in the figure. The bottom wire carries a current of 6.0 A. Point C is at the midpoint between the wires and point O is a distance 0.50d below the 6-A wire as suggested in the figure. The total magnetic field at point O is zero tesla.
-Determine the value of the current, I, in the top wire.

A) 2 A
B) 3 A
C) 6 A
D) 18 A
E) This cannot be determined since the value of d is not specified.