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 it is cold, then Karen is not writing a letter. It is cold if and only if Barbara is sad. Karen is writing a letter.
It is not cold can be inferred via the modus tollens rule. If it is cold, then Barbara is sad can be inferred via the biconditional elimination rule. If Barbara were sad, then it is cold can be inferred via the biconditional elimination rule.