What is the magnetic flux density at distance r due to a long conductor carrying current of I?

• A. \(\frac{{\mu I}}{{4\pi r}}\)
• B. \(\frac{{\mu I}}{{\pi r}}\)
• C. \(\frac{{4\pi \mu I}}{r}\)
• D. \(\frac{{\mu I}}{{2\pi r}}\)