The radii of the ends of a frustum of a cone of height $h \text{ cm}$ are $r_{1} \text{ cm}$ and $r_{2} \text{ cm}$. The volume in $\text{cm}^{3}$ of the frustum of the cone is:

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