Find parametric equations and a parameter interval for the motion of a particle that starts at $(a, 0)$ and traces the ellipse $\left(x^{2} / a^{2}\right)+\left(y^{2} / b^{2}\right)=1$
a. once clockwise.
b. once counterclockwise.
c. twice clockwise.
d. twice counterclockwise.
(As in Exercise $19,$ there are many correct answers.)