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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

(a) $3\sqrt{3}:8$                      (b) $16:9\sqrt{3}$

(c) 4:9                           (d) $2\sqrt{3}:9$