A magnet freely suspended in a vibration magnetometer makes $40$ oscillations per minute at place 'A' and $20$ oscillations per minute at a place 'B'. If the horizontal component of earth's maagnetic field at 'A' is $36 \times 10^{-6}T$, then its value at 'B' is :

Question

( A ) $288 \times 10^{-6} T$
( B ) $9 \times 10^{-6} T$
( C ) $144 \times 10^{-6} T$
( D ) $30 \times 10^{-6} T$