$A$ semi-circular arc of radius $r$ and a straight wire along the diameter,both are carrying the same current $i$. Find out the magnetic force per unit length on the small element $P$,which is at the center of the diameter.

• A
  $\left(\frac{\mu_{0} i^{2}}{4 r}\right)$
• B
  $\left(\frac{\mu_{0} i^{2}}{2 r}\right)$
• C
  $\left(\frac{\mu_{0} i^{2}}{r}\right)$
• D
  $\left(\frac{2 \mu_{0} i^{2}}{r}\right)$