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

The force per unit length between two current-carrying wires is expressed as F/L = µ₀/2πd) 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) µ_{0 .}
B) F/l.
C) .
D) .
E) FI²/l.