A magnetic compass needle oscillates 30 times per minute at a place where the dip is $45^{\circ}$, and 40 times per minute where the dip is $30^{\circ}$. If $\mathrm{B}_1$ and $\mathrm{B}_2$ are respectively the total magnetic field due to the earth at the two places, then the ratio $B_1 / B_2$ is best given by :
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