A magnet is suspended in such a way that it oscillates in the horizontal plane. It makes 20 oscillations per minute at a plane where dip angle is \[30{}^\circ \] and 15 oscillations per minute at a place where dip angle is \[60{}^\circ \]. The ratio of earth's magnetic field at two places is

• A. \[3\sqrt{3:}8\]   
• B. \[16:9\sqrt{3}\]
• C. \[4:9\]               
• D. \[2\sqrt{2}:3\]