A \( \mathrm{B}=\frac{\mu_{0} \mathrm{I}}{2 \pi \mathrm{a}} \)
B \( \mathrm{B}=\frac{\mu_{0} \mathrm{I}}{4 \pi \mathrm{a}} \)
C \( \mathrm{B}=\frac{2 \mu_{0} \mathrm{I}}{\pi \mathrm{a}} \)
D \( \mathrm{B}=\frac{4 \mu_{0} \mathrm{I}}{\pi \mathrm{a}} \)