The volume of a frustum of a cone with height \(h\) and radii \(r_1\), \(r_2\) of its circular ends is
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. \(\frac{1}{3}\pi h(r_1^2 + r_2^2 - r_1 r_2)\)
D. \(\pi l(r_1 + r_2)\)