$\begin {array} {1 1} (a)\;A\: point\: on\: any\: radial\: line & \quad (b)\;Any \; point\: on\: the\: axis \\ (c)\;The\: centre\: of\: the\: coil & \quad (d)\;Any\: point\: on\: the \: circumference \end {array}$
