a torus of revolution (doughnut) is obtained by rotating a circle c in the xz plane about the z-axis. suppose that c has a radius r and center (r, 0, 0). (a) (5 points) find a parametrization r(u, v) of the torus. specify the set d in which (u, v) must lie. hint: you can choose let u represent the angle that the line from the point r(u, v) on the torus to the center of the rotated circle form with the xy-plane, and let v denote the angle formed by the line from the point r(u, v) on the torus to the origin with the positive x-axis. see figures below.