CPM Homework Help : PC Problem 10-101

Give an example of:

Hint:

Any set of equations where $x = x\left(t\right)$ and $y = y\left(t\right)$ .

A graph that can be represented by parametric equations, but is not of the form $y = f\left(x\right)$ .
A graph that can be represented in the form $y = f\left(x\right)$ , but not by parametric equations.
Answer (b):
This is not possible. Why?