The magnetic field $($B$)$ at a distance $\text{'}r\text{'}$ from a long, straight conductor carrying a current $\text{'}$ I $\text{'}$ has a magnitude of

${}_0\frac{\text{I}}{6}r$

None

${}_0\frac{\text{I}}{2}r$

${}_0\frac{\text{I}}{4}r$