A Helmholtz coil has pair of loops, each with N turns and radius R. They are placed coaxially at distance R and the same current I flows through the loops in the same direction. The magnitude of magnetic field at P, midway between the centres A and C, is given by (Refer to figure):

A

$$\frac{4N\mu_0 I}{5^{3/2}R}$$

B

$$\frac{8N\mu_0 I}{5^{3/2}R}$$

C

$$\frac{4N\mu_0 I}{5^{1/2}R}$$

D

$$\frac{8N\mu_0 I}{5^{1/2}R}$$