Let the moment of inertia of a hollow cylinder of length 30 cm (inner radius 10 cm and outer radius 20 cm), about its axis be I. The radius of a thin cylinder of the same mass such that its moment of inertia about its axis is also I, is:\\
(1) 16 cm\\
(2) 14 cm\\
(3) 12 cm\\
(4) 18 cm