At a place on earth, horizontal component of earth's magnetic field is \[{{B}_{1}}\] and vertical component of earth's magnetic field is \[{{B}_{2}}\]. If a magnetic needle is kept vertical, in a plane making angle \[\alpha \] with the horizontal component of magnetic field, then square of time period of oscillation of needle when slightly distributed is proportional to

• A. \[\frac{1}{\sqrt{{{B}_{1}}\,\cos \,\alpha }}\]
• B. \[\frac{1}{\sqrt{{{B}_{2}}}}\]
• C. \[\frac{1}{\sqrt{{{({{B}_{1\,}}\cos \,\alpha )}^{2}}+B_{2}^{2}}}\]
• D. infinite