The magnetic field `vec(dB)` due to a small current element `vec(dl)` carrying a current I at a distance `vecr` is given as

Options

A. `vec(dB) = (mu_0)/(4pi) I[(vec(dI) xx vec(r))/(r)]`
B. `vec(dB) = (mu_0)/(4pi) I^2[(vec(dl) xx vec(r))/(r)]`
C. `vec(dB) = (mu_0)/(4pi) I^2[(vec(dl) xx vec(r))/(r^2)]`
D. `vec(dB) = (mu_0)/(4pi) I[(vec(dl) xx vec(r))/(r^3)]`

Answer

D