The volume of a frustum of a cone is equal to $\ldots \ldots \ldots . .$

• A
  $\pi h[r_{1}^{2} + r_{2}^{2} + r_{1}r_{2}]$
• B
  $\frac{1}{3} \pi h[r_{1}^{2} + r_{2}^{2} + r_{1}r_{2}]$
• C
  $\frac{1}{3} \pi h[r_{1} + r_{2} + r_{1}r_{2}]$
• D
  $\pi h[r_{1} + r_{2} + r_{1}r_{2}]$