8. Figure $P 1.8$ shows a frustum of a cone. Of the following mensuration (geometrical) expressions, which describes
(i) the total circumference of the flat circular faces,
(ii) the volume, and (iii) the area of the curved surface?
(a) $\pi\left(r_{1}+r_{2}\right)\left[h^{2}+\left(r_{2}-r_{1}\right)^{2}\right]^{1 / 2}$
(b) $2 \pi\left(r_{1}+r_{2}\right)$
(c) $\pi h\left(r_{1}^{2}+r_{1} r_{2}+r_{2}^{2}\right) / 3$
