Two thin long parallel wires,separated by a distance $d$,carry a current of $I$ $A$ in the same direction. They will:

• A
  attract each other with a force of $\mu_0I^2/2 \pi d^2$ per unit length.
• B
  repel each other with a force of $\mu_0I^2/2 \pi d^2$ per unit length.
• C
  attract each other with a force of $\mu_0I^2/2 \pi d$ per unit length.
• D
  repel each other with a force of $\mu_0I^2/2 \pi d$ per unit length.