The Force Per Unit Length Between Two Current-Carrying Wires Is $\mu$

The force per unit length between two current-carrying wires is expressed as
F/l = ( $\mu$ ₀/2 $\pi$ d) I²
Where I is the current, d the separation of the wires, and l the length of each wire. A plot of force per unit length versus I² gives a straight line, the slope of which is

A) $\mu$ ₀
B) F/l
C) (2 $\pi$ d/ $\mu$ ₀) ^1/2
D) $\mu$ ₀/2 $\pi$ d
E) FI²/l