The following is a system of three equations in only two variables.
$$\left\{\begin{array}{r}
x-y=1 \\
x+y=1 \\
2 x-y=1
\end{array}\right.$$
(a) Graph the solution of each of these equations.
(b) Is there a single point at which all three lines intersect?
(c) Is there one ordered pair $(x, y)$ that satisfies all three equations? Why or why not?