In the arrangement shown, magnetic induction at the centre O is

• A

  ${\mathrm{\mu }}_{\mathrm{o}}\mathrm{i}/16\mathrm{R}$

• B

  ${\mathrm{\mu }}_{\mathrm{o}}\mathrm{i}/24\mathrm{R}$

• C

  $5{\mathrm{\mu }}_{\mathrm{o}}\mathrm{i}/16\mathrm{R}$

• D

  ${\mathrm{\mu }}_{\mathrm{o}}\mathrm{i}/12\mathrm{R}$