What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Name the inference rule being used: If Mary takes a plane, then Charles is a sailor. Mary is not taking a plane. If Charles were a sailor, then Mary is taking a plane.
Charles is not a sailor can be inferred via the modus tollens rule. Mary is taking a plane if and only if Charles is a sailor can be inferred via the biconditional introduction rule. Charles is a sailor if and only if Mary is taking a plane can be inferred via the biconditional introduction rule.