$A$ hollow tilted cylindrical vessel of negligible mass rests on a horizontal plane as shown. The diameter of the base is $a$ and the side of the cylinder makes an angle $\theta$ with the horizontal. Water is then slowly poured into the cylinder. The cylinder topples over when the water reaches a certain height $h$,given by

• A
  $h=2 a \tan \theta$
• B
  $h=a \tan ^2 \theta$
• C
  $h=a \tan \theta$
• D
  $h=\frac{a}{2} \tan \theta$