Two long conductors, separated by a distance d carry currents I1 and I2 in the same direction. They exert a force F on each other. Now the current in one of them is increased to two times and its direction is reversed. The distance is also increased to 3d. The new value of force between them is
(a) −2 F
(b) F / 3
(c) (−2 F) / 3
(d) −F / 3
