One insulated conductor from a household extension cord has a mass per unit length of $\mu$. $A$ section of this conductor is held under tension between two clamps. $A$ subsection is located in a magnetic field of magnitude $B$ directed perpendicular to the length of the cord. When the cord carries an $AC$ current of $i$ at a frequency of $f$,it vibrates in resonance in its simplest standing-wave vibration state. Determine the relationship that must be satisfied between the separation $d$ of the clamps and the tension $T$ in the cord.

• A
  $T = 4\mu f^2 d^2$
• B
  $T = 2\mu f^2 d^2$
• C
  $T = \frac{\mu f^2 d^2}{2}$
• D
  $T = \frac{\mu f^2 d^2}{4}$