(4) (15 pts) Consider the following arguments. If an argument is valid, then present a proof sequence; otherwise, prove that the argument is invalid. You are forbidden to use truth tables to justify your answers (but, you may use them otherwise). ((p V q) r) (4) ( > ) V (q >)) (4b) ((q r) ^ (p (q V r))) (p r) (4c) ((p (q ^ r)) ^ (3 F) ^ (s 7)) (t p)