A cument is passed in an equilateral triangle of side l and made of a uniform conducting wire as shown in fig. The magnetic induction at centre O will be

• A

  zero 

• B

  $\sqrt{3}{\mathrm{\mu }}_0\mathrm{i}/2\mathrm{\pi l}$

• C

  $\frac{3\sqrt{3}{\mathrm{\mu }}_0\mathrm{i}}{4\mathrm{\pi l}}$

• D

  $\frac{9{\mathrm{\mu }}_0\mathrm{i}}{4\mathrm{\pi l}}$