In earth's magnetic field $\mathrm{B}_{\mathrm{H}}$, if the frequency of oscillation of a magnetic needle is $\mathrm{n}$, then (a) $n \infty B_{H}$ (b) $n^{2} \propto B_{H}$ (c) $n \infty B_{H^{2}}$ (d) $n^{2} \infty \frac{1}{B_{H}}$