A magnetic compass needle oscillates $30$ times per minute at a place where the dip is $45°$ and $40$ times per minute where the dip is $30°$. If $B_1$ and $B_2$ are the net magnetic fields due to the earth at the two places respectively, then the ratio ${\mathrm{B}}_1/{\mathrm{B}}_2$ is approximately equal to

• A

  $3.6$
• B

  $1.8$
• C

  $1.2$
• Correct answer
  D

  $0.7$

Hence, the correct answer is option: 'D'.