A circular coil of radius `R` carries an electric current. The magnetic field due to the coil at a point on the axis of the coil located at a distance `r` from the centre of the coil, such that `r gtgt R`, varies as
A. `1//r`
B. `1//r^(3//2)`
C. `1//r^(2)`
D. `1//r^(3)`