Consider the linear programming problem
$$
\begin{aligned}
\operatorname{minimize} & c_1 x+c_2 y \\
\text { subject to } & x+y \geq 6 \\
& 2 x+3 y \geq 5 \\
& x \geq 0, y \geq 0
\end{aligned}
$$
where $c_1$ and $c_2$ are some real numbers not both equal to zero.
a. Give an example of the coefficient values $c_1$ and $c_2$ for which the problem has a unique optimal solution.
b. Give an example of the coefficient values $c_1$ and $c_2$ for which the problem has infinitely many optimal solutions.
c. Give an example of the coefficient values $c_1$ and $c_2$ for which the problem does not have an optimal solution.