A compound proposition is satisfiable if there is an assignment of truth values to its variables that makes it true. For example, p q is true when p = T and q = T: thus, p q is satisfiable. When no such assignment exists, the compound proposition is unsatisfiable. Explain why a compound proposition is unsatisfiable if and only if its negation is a tautology. Give an example of an unsatisfiable compound proposition with two variables, p and q. Justify your answer.