A spherical bob hangs from a light inextensible string and moves in a horizontal circle with string making angle $\theta$ with the vertical. The length $l$ of the string is then very slowly increased so that the motion is circular at all times to a good approximation. If $h$ is the height from center of the circle to the pivot and $r$ is the radius of the circle then $h \propto r^a$. The value of $a$ is

• A

  $3$
• B

  $1$
• C

  $2$
• Correct answer
  D

  $4$

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