Figure shows a rough track, a portion of which is in the form of a cylinder of radius R. with what minimum linear speed should a sphere of radius r be set rolling on the horizontal part so that it completes goes round the circle on the cylindrical part?

Question

 

 

$$\begin {array} {1 1} (a)\;\sqrt {\frac{27}{7}(R-r)} & \quad (b)\;\sqrt{\frac{27}{5}g(R-r)} \\ (c)\;\sqrt {\frac{7}{5}g(R+r)} & \quad  (d)\;\sqrt {\frac{17}{5}g(R-r} \end {array}$$