Mass is non- uniformly distributed on the circumference of a ring of radius a and centre at the origin. Let b be the distance of the centre of mass from the origin. Then

Question

$$\begin {array} {1 1} (a)\;b=a & \quad (b)\;0 \leq b \leq a \\ (c)\;b < a & \quad  (d)\;b > a \end {array}$$