An infinite wire has a circular bend of radius a, and carrying a current I as shown in figure. The magnitude of magnetic field at the origin O of the arc is given by :

A

$$\dfrac{\mu_0 I}{4\pi a}\left[\dfrac{\pi}{2}+1\right]$$

B

$$\dfrac{\mu_0 I}{4\pi a}\left[\dfrac{3\pi}{2}+2\right]$$

C

$$\dfrac{\mu_0 I}{2\pi a}\left[\dfrac{\pi}{2}+2\right]$$

D

$$\dfrac{\mu_0 I}{4\pi a}\left[\dfrac{3\pi}{2}+1\right]$$