$A$ long curved conductor carries a current $I$. $A$ small current element of length $dl$ on the wire induces a magnetic field at a point away from the current element. If the position vector between the current element and the point is $\vec{r}$,making an angle $\theta$ with the current element,then the induced magnetic field density $d\vec{B}$ at the point is $(\mu_0 = \text{permeability of free space})$:

• A
  $\frac{\mu_0 I d\vec{l} \times \vec{r}}{4 \pi r^3}$ (perpendicular to the current element $d\vec{l}$)
• B
  $\frac{\mu_0 I \vec{r} \times d\vec{l}}{4 \pi r^3}$ (perpendicular to the current element $d\vec{l}$)
• C
  $\frac{\mu_0 I d\vec{l} \times \vec{r}}{4 \pi r^2}$ (perpendicular to the plane containing the current element and position vector $\vec{r}$)
• D
  $\frac{\mu_0 I d\vec{l} \times \vec{r}}{4 \pi r^3}$ (perpendicular to the plane containing the current element and position vector $\vec{r}$)