$A$ deflection magnetometer is adjusted and a magnet of magnetic moment $M$ is placed on it in the usual manner and the observed deflection is $\theta$. The period of oscillation of the needle before settling of the deflection is $T$. When the magnet is removed, the period of oscillation of the needle is $T_0$ before settling to $0^{\circ}-0^{\circ}$. If the earth's horizontal magnetic field is $B_H$, the relation between $T$ and $T_0$ is

• A
  $T^2=T_0^2 \cos \theta$
• B
  $T^2=\frac{T_0^2}{\cos \theta}$
• C
  $T=T_0 \cos \theta$
• D
  $T=\frac{T_0}{\cos \theta}$