Two thin, long, parallel wires, separated by a distance d carry a current of i ampere in the same direction. They will

• A

  Attract each other with a force of ${\mathrm{\mu }}_0{\mathrm{i}}^2/\left(2{\mathrm{\pi d}}^2\right)$ per unit length

• B

  Repel each other with a force of ${\mathrm{\mu }}_0{\mathrm{i}}^2/\left(2{\mathrm{\pi d}}^2\right)$ per unit length

• C

  Attract each other with a force of ${\mathrm{\mu }}_0{\mathrm{i}}^2/\left(2\mathrm{\pi d}\right)$ per unit length

• D

  Repel each other with a force of ${\mathrm{\mu }}_0{\mathrm{i}}^2/\left(2\mathrm{\pi d}\right)$ per unit length