The unit vectors $\hat{i}, \hat{j},$ and $\hat{k}$ are as shown below. What will be the magnetic field at $O$ in the following figure?

• A
  $\frac{\mu_0}{4\pi} \frac{i}{a} \left( 2 - \frac{\pi}{2} \right) \hat{j}$
• B
  $\frac{\mu_0}{4\pi} \frac{i}{a} \left( 2 + \frac{\pi}{2} \right) \hat{j}$
• C
  $\frac{\mu_0}{4\pi} \frac{i}{a} \left( 2 + \frac{\pi}{2} \right) \hat{i}$
• D
  $\frac{\mu_0}{4\pi} \frac{i}{a} \left( 2 + \frac{\pi}{2} \right) \hat{k}$