A magnet is suspended in such a way that it oscillates in the horizontal plane. It makes 20 oscillations per minute at a place where dip angle is 30o and 15 oscillations per minute at a place where dip angle is 60o. The ratio of total earth's magnetic field at the two places is [MP PMT 1991; BHU 1997]

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