I.   Consider the following argument forms, and determine which are
            valid.

           A.   P -> Q
                ~Q
                ------
                ~P

           B.   P v Q
                P
                -----
                Q

           C.   P
                -----
                P v Q

           D.   P -> Q
                Q
                ------
                P

           E.   P -> Q
                Q -> R
                ------
                P -> R

           F.   (P -> Q) & (R -> S)
                P v R
                -------------------
                Q v S

           G.   P -> Q
                ~P
                ------
                ~Q