# Calculate the magnetic field between the plates at a point $15\: cm$ from the axis.
$\begin {array} {1 1} (a)\;0 & \quad (b)\;2 \times 10^{-7}T \\ (c)\;1.35 \times 10^{-7}T & \quad (d)\;2.5 \times 10^{-7}T \end {array}$