18. The wire shown in the right figure is infinitely long. It is bent at right angle and carries current $I$. Find the magnitude of the magnetic field at point $P$ which is at a distance $a$ from the wire.
(A)$\frac{\mu_0 I}{4\pi a} (1 + \sqrt{2})$
(B)$\frac{\mu_0 I}{2\pi a} (1 + \sqrt{2})$
(C)$\frac{\sqrt{2} \mu_0 I}{4\pi a}$
(D)$\frac{\mu_0 I}{2\pi a}$

(E)None of the above

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